Anything, really. Including what the rest of us think the other's roles are, which is quite vital to figuring out who to noose, really. Though it is mostly guesswork without a (real) Knight confirming roles, hanging the right person can mean the difference between a Villager win and a Mafia win. :P of course, actually revealing what those roles are is a fool's move. The Mafia will know who the other Mafia are, but not what everyone else is. Revealing who you are or aren't will make the guesswork for nailing a Knight or a Cleric a lot easier-or, if you are Mafia, get you noosed pretty much immediately. :P

So, in short, figuring out who everyone is? That's half the goal. The other half is acting on it.

EDIT: What?! I'm not your friend, Velo?! ;_; Dude, that's harsh...