Perhaps, if it does exist, it is used for some authentication of sorts, when you initiate games online with it ?
I'm not sure how it works, I dont have one, but it seems to me connections through a medium, like some big gaming server, instead of direct connection between the two isps and hence the dreamcasts, would be a waste of bandwith.
The authentication could be to avoid country mixups, lets say it was Sonic adventure japanese and american ( heh if that game cannot use gaming online, forgive me on that, i dunno virtually anything about it )
Username: SonicAdventure
Password: here the japanese game connects to the american, hence tries a password like 'jappa'.
It cannot connect, the connection dies, and the game gives you an error. This could also work with version differences, if you can download some game-patches or similar that would change the two games.
that's about it
-scar