Facial moisturizers can enhance the functioning of the stratum corneum, the uppermost layer of skin. The stratum corneum is made up of fat, oil, and skin cells that help keep moisture in the skin. Applying a moisturizer to your face can increase the moisture level of the stratum corneum, prevent skin from drying out, and, if the moisturizer contains a sunscreen, also protect against sun damage.
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
Quoted from Dr. Ellen Marmur, Dermatologist