The cable just connects the gameboy advance to the cube for certain games. Like in Wind Waker, you can have a friend use the gameboy advance as Tingle and follow you and give you items when you run out and such. It will also let you tranfer your R/S Pokemon to Pokemon Colosseum. There are other games that use it like FF CC, but it doesn't function as a controller or allow playing of Gameboy advance games.
p.s- shouldn't this be in Video Games discussion?