This cable, when used with specially designed games, allows you to connect the Game Boy Advance or Game Boy Advance SP to the Nintendo Game Cube. The features available vary depending on the game - from exchanging data to unlocking new levels, to using the Game Boy Advance or Game Boy Advance SP as a supplemental game screen or controller!
I had this when i was younger, but while moving houses I lost it. i found it very useful for certain games I played, such as The Legend of Zelda: Wind Waker. It gives games that are compatible new parts, that can be very interesting, annd helpful.