Social time: last.fm

Alright, social time:

Why would one want to use last.fm? I'm sure there are reasons, I just don't know what they are.

But I'd like to. I've got a last.fm account, but no real idea why. I think I use it as a "junk I've listened to" list, currently; but I don't think that's a 'good' use of it/ think that's under-using it.