Well, it does a few things. For one thing, when ever one of your buddies is online, their name will have a "+" by it. It just makes them stand out a bit more in the big mess of names at the bottom of the page.
Its also good for PMs. all you have to do is click there name to send a PM, rather than having to type it out. (Sinse you have to spell the name exactly, this is quite handy, especially if your buddy has a hard to spell name.) EDIT: No, thats not true. I think it was like that in the old forum, though. At least you have the correct spelling of your buddies name on hand.
I think it can do a few other things, but to be honest, I never really use it. I only have something like 3 or 4 buddies.