Natural human hair get moist by sebum, which is the product of the microscopic glands in the skin that secrete an oily/waxy matter. Sebum lubricates hair and provides the hair shaft with protection and water-resistance.
If the cuticle layer is damaged, the natural hair nourishment or sebum, has to fill missing shingles and run short. Lack of sebum makes hair shaft more vulnerable to water and mechanical damage. The more damaged and kinkier the hair, the more difficulty the sebum will travel down the hair, therefore the drier or dull the hair looks.
Sebum has lots of organic similarity with oils. Oil coats the hair by latching onto cracks and holes in the hair shaft. Damaged hair has a scaly surface which allows the oil to penetrate the hair as it slides down the hair and slips into those cracks.