Your skin is constantly getting attacked and cut in many ways. It's incredibly important. Viruses and bacteria try to get in, you bump into things and lose skin cells, the sun's light kills cells, and all of this happens without you knowing. Skin cells have to continually repair and reproduce to keep you patched up and functional as the first line of defense against everything. It's a very effective system!

Skin gets damaged, and wears off. Unless it were constantly being replaced, you would soon be without skin, which would cause you to die soon after. Skin cells undergo mitosis to replace lost skin, so that you never run out.