I find when starting a new project that I can remember a lot of stuff, too much really.
I also find that at some point I will forget much of what I have learned if I'm not actively working on that project.
Thus I take a lot of notes, because nothing is more frusterating than not remembering how you did something or why you did it in the first place.
This Wiki allows me to create, categorize, and access my notes from just about anywhere. Which is great because it sucks when you can't remember how to do something and you know you've made notes about it, but can't remember where you put them or don't have access to them when you need them.