When it comes to the GC/GBA Cable. You wouldnt think that something this inexpensive could make a difference in the way you play video games. But it does it unlocks secrets of all kinds in Gamecube games, sometimes it gives you access to secret areas/maps/items and various other thing.
Depending on the game and what features it opens up. Plus it is cheap to get and once you've gotten this wonderful item. You will wonder why you didnt get it sooner. I know from my own experience.