This can be very useful when you want to check who is using an image that you've own. In a small search I could find some sites using my panoramio pictures, without asking me anything. So they are doing business using copyrighted images.
I come across many other uses, for instance you need a better resolution image from a small icon you need, tineye will for sure help you on this!
output from tineye:
Share your uses on comments please! Have fun.