Originally Posted by NorthGeorgiaHawg

It locks your detachable hardware onto your bike, to make it *slightly* more difficult for people to steal off of your bike. Someone who really wants to steal your detachable can stick a screwdriver in between the rack and the cam and pop the whole thing off pretty easily. It only thwarts lazy thieves...

I have one on my detachable Tour Pak rack, but it's a bitch to get the long key up under there to unlock it. I never unlock it anyway though - I only got the detachable rack because the solid mount racks that would fit my bike have long been out of production and are very hard to find, and it looks better, too.