To keep track of your listening habits and building a library of artists and also is a way to know bands according to your tastes. That's the main purpose, then comes building events charts of the bands and creating some sort of wikis and build your favorite bands' profile on the site by uploading your favorite photos and shit. It's unique.
On the other hand is like any other social network, you'll get to know tons of people with similar taste to talk with.
I have a 9-year old account there and holy shit, it's amazing how it never fails to recommend good bands.