Stem cells are unspecialized cells which divide to become many different types of specialized cells during embryonic development.  Some stem cells remain active in the adult human and continuously produce cells called "transient amplifying cells" which are the source for new blood, skin and bone cells.  It would be less complex and cost less energy if existing skin or fat or bone cells made their replacements by normal cell division. So why are many cells replaced by this less efficient method?  The answer may surprise you.  Uplook Magazine August-September 2008, p4.  References:  J. Panno, Stem Cell Research: Medical applications and ethical controversy (New York, NY: Facts on File, Inc, 2005).  J.W. Pepper, et al. Animal Cell Differentiation Patterns Suppress Somatic Evolution, Plos Computational Biology.  Vol3(12) e250, 2532-22545, 2007.