I used to do this for D&D games and such. We don't use anything like it for AW though, putting in the effort outside of the game pays its own dividends. Getting an image of yourself and putting it on display just helps everyone see the character the way you do, which only enhances that interaction. Writing a journal, or otherwise provides deeper context for your decisions, and with the others able to read it, they are prompted to get to know your character, or interact with your character in a way they might never have had time to do during play.
Also, because writing journals and doing off scene creation stuff with the MC is a way to have a more, mutually agreeable final image. Nothing like an MC giving that forced smile when you say what you think is awesome, and they have to try to agree. Much better when both people are already down with it, and they've bounced that ball back and forth a few times to get an even more honed tale.
However, a word of caution. Players that end up doing all the world building can disenfranchise the other players that dont. Say one guys has been thinking about something that happened in one of the games, and is waiting for a good time to bring up what he thinks that was all about--when another player together with the MC decides its something else in the middle of the week. Not cool. These things should generally be created in front of everyone with opportunity for the others to weigh in, or color in those lines at the same time.
So. I have to say, I strongly recommend you do not give additional benefits to players that are already giving themselves additional benefits.