Online Gaming (On WiFi marked games.)
Downloads of classic and new games (With cost)
Browse the Internet
And so on.
In gaming the friend codes allow you to play a WiFi game with a friend if both of you exchange them, as for the Wii system code you can exchange them with friends to chat trough the Wii, and register friends to newer WiFi games (like GH WoR)
But all that I just wrote is pretty obvious if you ask me.