It's so you can share what you've been listening to with a forum full of strangers basically. Or give your link to someone so you can see how compatible you are. I kind of forget about it for the most part, visit it every week. Their recommendations are fine, but there are countless other sites that have that. (You know the ones where you put in an artist and it branches off similar artists.) Well, you can listen to music on Last FM as well but I never bother with it. I have iTunes for that.
Rate Your Music > Last FM.