from what i can tell...
if your involved in a big board that has well over 100,000 members (there are a few out there)....then a buddy list would be good for private messaging (which is currently disabled on this site)...so when you login into the board, you can go to your buddy list and see who is and who isnt online (or active with the board at the current moment)...that way, knowing that person is looking at the board and is active...you could send a PM