Some months ago, I asked What Do Rules Ever Do For Us? Here, I’m going to ask “What do rulebooks do for us”? I’m asking not about rules (abstract things that could be just in the players’ heads) but about actual rules texts.
Rulebooks can sell
Rulebooks can start by inspiring one person to want to play. If there is a GM, it will often be this person. This was my experience with Zweihander — reading the book (and, particularly, looking at the pictures) made me want to run it. I wanted to play in that world.
Rulebooks can then help to encourage people to play. The zealot from above can wave the book at players. Those players can look at the art, read the prose, scan down lists of abilities (or insanities) and come to feel that they want to play, too.
Early on in extended play, the quality of the rulebook materials can build player confidence that the rules are good. Many rulesets are badly broken; many others have nontrivial problems. Almost everybody hits apparent problems when they’re first using a new game. A rulebook with good writing, good art, and few text errors is an easily-observable sign that the developers had control of something about the game. And that gives some confidence in the harder-to-assess quality of whether the rules will give you good results.
During multi-session play, when interest flags, rulebooks can help sustain and rekindle enthusiasm. Players who are flagging can look at the book between sessions, see that art again, see the abilities at their next level/tier/advance and imagine using them, see the rules for infected wounds and imagine getting one.
Rulesets and rulebooks can signal personality traits . Apocalypse World and Lamentations of the Flame Princess, with their bizarre, often sexualised and often groteseque artwork, signal openness to experience.
Rulebooks can inspire
Rulebooks can provide seed content, getting GMs (and often players) started in setting design.
Rulebooks can provide fat game resources — library content in the form of monsters, spells, magic items, encounter lists. A lot of people live for that.
Rulebooks can instruct
Rulebooks can teach rules.
Beyond the rules, rulebooks can provide guidance.
Rulebooks can set expectations, through pictures, through prose, through conversational boxouts from the designer. Apocalypse World does the latter a great deal (and doesn’t bother with the boxouts). If you’re lucky, rulebooks can thereby help maintain consistency of expectations across a play group, especially in the first few sessions with the game.
Rulebooks can assist
Rulebooks can provide a rules reference, although that use is often in conflict with their teaching and guidance roles.
Rulebooks can bind the aforementioned fat game resources into a form that lets you find them when you need them, rather than you having to hunt through a stack of printed blog posts.
Rulebooks can be their own reward
Rulebooks can provide aesthetic pleasure, from their words, from their art, from their touch and from their smell.
 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.