- It has a come across a bunch of images without any tags or meta data and is looking for ways to index them
- It is testing ways to get human brain to do the job of cheap computers by giving them interesting incentives ( this is no different from how we train monkeys and pigeons to do a particular work). You get them to play a game, and reward them with a cookie at the end.
- Or, as I suspect, google is testing an image recognition software and need human input to validate the images.
Either way, this is a very cool idea and I'm pretty sure everyone else will be doing the same thing in no time.