If you are a facebook app developer, your ears should perk up when people say things like "what's the use case?" or "why would I use this instead of xxx?". I think the general solution is to have a good answer to the question, then tell your potential users about that answer -- for example, on the application description page. "Collaborate easily on class projects! Write a story with some of your friends!" Or whatever.

I didn't mention it in what I said, but I don't think I would use your app either -- which I guess is the ultimate question, the one that matters.