Creatine is taken as it's an arginine metabolite - arginine is an essential amino acid which must be obtained through the diet, since it cannot be made by the body. In some clinical circumstances (e.g., burn injury, sepsis) in which the demand for arginine cannot be fully met by de novo synthesis and normal dietary intake, exogenous arginine has been shown to facilitate the maintenance of lean body mass and functional capacity. For the case of creatine, there have been reports
that taking it as a supplement facilitates an increase in anaerobic work capacity and muscle mass BUT ONLY when accompanied by resistance training programs. Taking it alone and sitting on your fat (or skinny) arse has had no effects whatsoever.
Still, even that data in clinical trials has been contentious.
If any of you are interested in *real science* data and not just shite fed to you by the TV, go and search the scientific archives:
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi