It's too hard to get a bunch of people together at the same time, in the same place, in order to practice and record music. And sure, you can email Garageband files around, but then everyone has to have the same software and it's a ton of work.
Bananajam enables you to record a track, then share it with your friends, who can layer in more tracks. So instead of going through the hassle of trying to get a group together in a physical location or going through the deep despair of never actually finishing a song, you can do it all right from your phone.
Rock stardom, here we come.