The surface of your skin has a layer called the acid mantle.  This layer is comprised of sweat and oil.  I know it sounds pretty unattractive but it isn’t.  It is however, very important.  This mix of oil and sweat serves two purposes helps your skin perform two of its primary functions:  moisture in, germs out.  The natural oil helps to hold the moisture in your skin keeping you from dehydrating.  The sweat layer makes the skin slightly acidic.  Bacteria and virus are mainly alkaline and do not fare well in an acidic environment.  So preserving this acid mantle as much as possible is a huge help to keeping your skin looking young and healthy.