“Because they do.”
For the first thirty-some-odd years of my life, “Because they do” would be how I answered the question “Why do finger tips get all wrinkled when you stay in water too long?” Never even thought to understand why it happens. Just accepted that being in water for an excessive amount of time made my hands look like old ballsacks and left it at that.
As it turns out, Science thinks this wrinkling effect is happening for a reason, because Science thinks everything happens for a reason. Stupid, Science, always ruining all the fun of not knowing.
So what is that reason? Grip. Science thinks the wrinkles — which are from blood vessels causing upper layers of skin to pucker — form on your hands and feet so they can grip things better in the wet conditions your body is currently in.