So you can tell how far away things are. Duh.
Actually, I was thinking of investing in a webcam or two, a USB dart launcher, and some servos to make a system that'd automatically fire darts at things. Maybe two webcams and one high-powered laser pointer, so you could point the laser at things and have it automatically calculate heading, distance, compensate for gravity, and fire.
It'd be near-impossible to get everything to work together just right, though. And I don't even know if the laser thing is possible at a distance.