It doesn't have to be detailed at all. The surprises come from the interactions and natural evolutions. But if a particular adventure is taking place, or if you plan on taking your character to a different town or whatever, it gives people the opportunity to get involved with that. Or, if somebody sees something taking place in the Official RP thread, it gives them a place to ask if they're welcome to just rock up and join. Like I said, it's basically just an informal chat where ideas are bounced around and people join up that way.