Tests your colour vision. If you're colour blind, it will tell you how colour blind you are, and what time of deficiency you have.
There is a Counting Crows joke in here somewhere, but all kidding aside, colour blindness is really, really common. If you've never checked, you probably would like to know definitively whether or not your among the millions of people who have defective cone cells in the eyeball region. And if you have to subject yourself to a vision, you might as well do it in a beautifully designed app like this one.