Why are skin cells undergoing mitosis continuously

Our skin cells divide rapidly in order to maintain a protective barrier against infection.The outer skin layer is called the epidermis and contains mostly dead cells that contain keratin.Keratin is a tough fibrous protein found in hair,skin and calluses and acts as a protective coating.The epidermis cells are constantly undergoing mitosis so the outer dead cells containing keratin are rapidly replaced as they fall off, which occurs after so many days.Then the newer living cells start producing keratin,lose their cellular contents,then die and the cycle goes on.The skin acts as a barrier to infection,therefore,it's constantly renewed and repaired.

Its either a) or b),wind and water erosion gradually widen the canyon