For those who think it's because of skin protection. See below. "Sebum Secretion and Sebaceous Lipids", Stewart et al, Dermatologic Clinics -- Vol. 1, No. 3, J

For those who think it's because of skin protection. See below. "Sebum Secretion and Sebaceous Lipids", Stewart et al, Dermatologic Clinics -- Vol. 1, No. 3, July 1983. Sebum is an oily substance that is secreted onto the skin surface from glands located in the dermis. Although a number of useful functions have been proposed for sebum, proof that sebum performs any of them is lacking. In furred mammals an essential function of sebum is to supply 7-dehydrocholesterol, which is converted to vitamin D by the action of sunlight and then ingested by the animal as it grooms itself. In man, however, the location of 7-dehydrocholesterol has been shown to be the epidermis rather than sebum. Sebum may act as a waterproofing agent for fur, but humans obviously have little need for this function. Kligman has specifically disproved the notions that sebum improves the barrier function of skin, that sebum helps to regulate the water content of the horny layer by forming emulsions with sweat, or that sebum on the skin surface is fungistatic or antibacterial.(21) Kligman regards the human sebaceous glands as 'living fossils' that lost their usefulness to our species as we lost our fur.(21) (21) Kligman, A. M.: The uses of sebum? In Montagna, W., Ellis, R. A., and Silver, A. F. (eds.): Advances in the Biology of Skin. Volume 4. Oxford, Pergamon Press, 1963.