It is just for the purpose of sending private messages through the Forums. If you add some people to your buddy list, then through your own profile you can see all their names together at a glance, making it a bit easier to click a name and send a private message to them, vs. searching for a post by them first to click their name from. If you have private messages turned off, then this feature doesn't matter. Doesn't effect anything else, and is really not that greatly helpful IMO, unless you send alot of them. I assume the 'ignore' feature means that those on the ignore list can't send you a private message.
To check for any private messages you have, just click on your own profile. It will also show any you have sent out, and whether that person has read it or not yet. There is also a feature you can turn on or off, where an e-mail will be sent to you also, letting you know a new private message has arrived. This is a bit redundant to have turned on, unless you keep forgetting to check your profile occasionally.
