It lets you view all games (in the past 2 weeks) of all the players on XBL playing Halo 2. It lets you show off your awesome stats for one game, and it also allows Bungie to closely moniter games for cheaters.
sign in here:
http://www.bungie.net/ (link is at the top. A .net e-mail, such as hotmail, is required.)
After that, go under My Stats (you don't have to have your XBL account connected to your .net email).
To the left, there will be a little clan/ GT search engine. Use that to find your or someone elses account and games.
EDIT: no, you don't need XBL, I dun think. You just need the .net e-mail.