Think of the skin's surface like the paint job on a car. If the paint is cracked, the metal underneath is left unprotected from the elements and quickly oxidizes and rusts. The same concept goes for your stratum corneum - once it's dry, brittle, or cracked, you've lost your shield and the skin is vulnerable to exposure from the outside and water evaporates from the dermis. You can get fissures on the skin, and infections can easily occur. When the barrier function is compromised for any reason (trauma, wind, dry air, too many chemicals on your skin), bacteria enters the dermis through the cracks on the surface. The stratum corneum contains approximately 30 percent water and lipids. Its water content is essential, even though it's made up of dead cells. If that Gore-tex dries out completely, it can't protect the layers of skin underneath. Similar to the shiny, beautiful coating on a car's exterior, putting on moisturizer not only makes the surface look pretty, it seals and protects what is beneath it.