I don’t think it’s as weird or confusing as you guys are making it sound. Lots of community sites have an option to add friends, and a forum is a community. Who really knows what it’s actually for. Basically I think just to display who you’re friends on the forum are, or to get in contact with them quickly. It can help build relationships by adding people that share common interests, etc.
Once a board gets big, it can be pretty hard to find one of their posts just to click on their username to get to your friends profile!
I think this is a really cool plugin idea! Is there anything out there that can do this?