People used to think that the wrinkles happened when the outer layer of our skin on our fingers and toes swelled up with water, but now we know that pruning is caused by blood vessels below the skin constricting.
It's an involuntary reaction to wet conditions by our autonomic nervous system, the same system that controls things like our breathing, heart rates and perspiration, things we don't have to consciously think about to do.
But why did our bodies, actually just our fingers and toes, evolve to react this way when wet?
You see, a few people started taking a closer look and thought, "hey, you know those wrinkles on our fingers and toes? They kind of look like tire treads, don't they?" So they began to test a hypothesis.
Maybe we wrinkle so, like tire treads, our fingers and toes can get better traction in wet conditions. Sure enough, our pruny digits do a better job of handling wet objects than dry fingers.
It's possible this nifty trick helped our ancestors get a better grip and sturdier footing in wet conditions. Thanks, science! Now I can kick butt at my next water balloon fight.