Some months ago, I asked What Do Rules Ever Do For Us? I asked, there, “Why use rules? Why not just freeform?” Under “rules”, I included those that were “RAW from a third party text, hand-crafted by the GM, or assembled by the play group through a democratic process”. Here, I’m going to zoom in on the first of those and ask “What do published rulesets do for us?”. I’m not interested, here, in things that any collection of rules can do — I’m interested in a what a set of rules carefully designed by a third party can do for you.
I’m not asking, here, about rulebooks per se, as texts or as physical artefacts — I’ve asked that elsewhere. I’m asking about the rules themselves, howsoever communicated and stored.
Context — isn’t that a silly question?
I know several rpg rulesets tolerably well. I have hacked some of them extensively. I have designed a few of my own games, whole-cloth. If I want to run a traditional rpg in a particularly setting or style, and I don’t know a ruleset that’s right for it, I can probably hack something to fit.
Yet I’m still interested in learning new published rulesets. Not just high-concept storygames with very distinctive rules — I’m up for learning quite traditional rpgs. I’m doing this with Zweihander right now.
Learning and using third party rules is hard work, though. I can sink hours into learning, more hours into GM prep (slowly, because I’m clumsy and awkward with the unfamiliar rules), and several hours at least into unsatisfying play (because it turns out I don’t actually enjoy this weird new game). My experiences with new games are decidedly mixed.
And, of course, many published rulesets have serious problems. For every ruleset that’s carefully design such that if you do what it says you’re likely to have a good time, there’s about… one, I’d guess, that if you actually do what it says you’ll end up in a mess. As Paul Beakley said recently:
“[Burning Empires] was a radical departure from the game I had run just prior, Exalted. It required I stick close to the rules as actual rules in a way that nothing I’d ever played before was even capable of doing. If you stick to Exalted’s text, well, your game will fall apart.”
I’m not sure that Burning Empires is a “good game” for many people. But, like the rest of Crane and co’s games, it means what it says. It doesn’t assume that you will patch and fudge and generally do all the work yourself. This is not uncommon, but it’s far from guaranteed.
I think I’m somewhat unusual in that I tend to start with a vision of a setting or situation, rather than wanting to play some particular game I’ve heard about. I play rpgs far more to create than to consume. Given a specific ruleset, I’ll very quickly have strong views on how I’d have written it, and will want to hack it to death and back. So I find published rulesets particularly frustrating. 
But, even given the above, I still do it. Why? Well…
Published rulesets can help you play
A published ruleset can get a beginner started. I suspect it’s hard to establish an enjoyable tabletop game without ever learning someone else’s rules.
That said, freeform play works for some people, and much online freeform (albeit of a GMless character rather than the GM’d forms I’m talking about here) doesn’t involve the kind of rules I’m talking about. E.g. one poster on rpg.net —
Sure, I started with freeform play. Particularly forum freeform, which you can still find on plenty of forums, as well as even on facebook and tumblr where there’s ‘character-focused’ freeform in bite-sized pieces talking about one specific character.
Published rulesets can save you a lot of design time. Game design is very hard. Many of us like complex rulesets, and they are particularly hard to design because they have many interacting parts. It’s easy to accidentally build in dominant strategies (e.g. “be a wizard”) or annoying properties (e.g. “fights take much longer at higher levels”). Similarly, coming up with fat game content is very time consuming.
That said, these days I’m very suspicious indeed about complex games, about whether they give me any actual benefit. Simple games are not so hard to design, or to hack from something you know, and they let you front-and-centre the things you care about (e.g. explicit player flags, resolution of social conflict) and competely exclude those you don’t (e.g. and the every popular but rarely used “damaging an inanimate object”).
Of course, I have basic competence in game design. If you lack design skills, or supporting skills like probability and stats, it may be just impractical to create any rulesets you want. Especially if the rulesets you want to play are complex ones. (This is a common view in the rpg.net thread I started on this post)
Extending the above, published rulesets can give you a formula for play that has worked well for others. Many game designers put great effort into their games, and sometimes they arrange extensive playtesting. Designers thus often learn a great detail about how their rules work, and how they fit together, and are thus able to improve their function.
In contrast, when you design your own rules just for your table, you are playtesting as you go. You learn about e.g. problems with powerful wizard PCs when a wizard in your PC group becomes powerful. If you’re busy and industrious, you might run two games in parallel (as I did with both StarCruiser and Beyond the Forest), but you probably won’t run ten.
Again, I have my doubts about this benefit. RPG design is thick with cargo cult practices, leading to rulesets swollen by subsystems and details that even the designers never use. But as Buggritall says on rpg.net, we’ve never been better in terms of try-before-you-buy.
And if, like me, you want to houserule and hack and change to your vision, the value of other’s experiences is much less. With every change you, their experiences become less relevant.
Published rulesets can provide novelty — the GM and players can be pleasantly surprised by how things interact, or by e.g. what magic item the treasure generator turns up. This is of course many-edged — some surprises are unwelcome, especially as the GM or players may have committed to situations or characters that now don’t make sense are much more or less dangerous.
You can be surprised by your own rules, too, but I’d wager a smaller proportion of such surprises are pleasant.
Published rulesets can serve as a security blanket, reassuring the players that even though right now they feel like they’re floundering — teetering on the edge of being boring, offensive and acrimonious — their game is actually going ok, and is going to go well. That confidence comes from knowing that many others have had good experiences with these rules before. In serious pursuits that kind of reassurance can be a problem, but for a leisure activity this is probably a good thing.
Published rulesets can help you design
In the simplest sense, rulesets can provide design ideas — rules, subsystems, dynamics, ways of wording. Kenco on rpg.net says they mostly engage with rulesets in this way, and I suspect they’re not unusual in that.
I suspect, however, that complete rulesets are not the most efficient way to communicate most design ideas. Most new ideas are just small changes to existing ones. They are more usefully expressed as a modification to some well-known existing ruleset. The OSR community is particularly good at this.
That said, rulesets can sometimes show you a way to combine ideas together. In particular, they can show you a novel rule or subsystems in a context that allows it to work. My favourite example of this is Dungeon World. DW encourages the GM to make up most of the world on the fly, partly by asking players to narrate what their PCs know about it, and it supports this by giving monsters and NPCs very minimal stats. It’s much harder to do the same in Pathfinder or even 5e.
Published rulesets can take you part of the way to the design you want. Sometimes I read a ruleset and want to make radical changes, but sometimes I just want TSoY style keys instead of a deadweight “give everyone 100xp each session so no-one gets jealous”.
Published rulesets can help you connect
Rulesets can provide access to online communities of people playing the same game. And in this hyperconnected age such communities can be extremely valuable, fixing rules holes, giving you free new material, and generally giving you access to the wisdom of the crowd. They can be rich in dickheads, too, but such is the world.
(Modulo that players of “the same game” may be using a different subset, or a different misreading, or be making very different assumptions in the spaces the rules leave open . And that any given community may turn out to be rich in dickheads.)
Tightly-focussed multi-ruleset communities, such as the OSR and its sub-groups, can have much the same effect. But for most forms of play there are no such communities — there are only ruleset-specific ones.
Rulesets can provide a handy signal for game style. If I know you’re playing Pathfinder, I’ve got a reasonable suspicion that you’re heavily into rules mastery, combat, character-build-optimisation, and following the GM’s plot. If you’re playing Burning Wheel, I can predict that you are similarly interested in rules mastery, but much more into character-driven emergent stories. If you’re playing Risus, I can be confident that you’re not into rules-mastery at all.
Rulesets can signal group affiliation. Using Labyrinth Lord signals that you are probably in the OSR, Burning Wheel suggests a Forge diaspora affiliation, and 5e suggests you are part of mainstream D&D.
Much like rulebooks, published rulesets can signal personality traits . For example, Burning Wheel is hard to understand, effortful to learn, known to be rigid and somewhat fragile to hacking — using it signals intelligence, conscientiousness and adaptability.
 As I said in the Rulebooks post, the best explanation of signalling I have to hand is David Chapman’s “Ethics” is advertising. It’s unfortunately about a great many other things as well — religion, ethics, and social class — which may or may not interest you.
 I say “somewhat” special because lots of other people do this, hacking and dropping rules devil-may-care. Lots of people seem to do this without realising it, just by being forgetful or by not taking the rules too seriously. Many end up playing their usual game with different fat and colour. Indeed, I wonder what proportion of groups playing traditional GM’d games ever play serious RAW, even when they try, and thus ever get the full alien experience of a new ruleset.