Creatine is a Natural Component of Skeletal Muscle
In fact, creatine is such an integral part of skeletal muscle that it originally derived its name from the Greek word for flesh, or kreas, from where it was isolated nearly two centuries ago (1835) by the French scientist and philosopher, Michel-Eugène Chevreul. Shortly afterwards (1847) the German scientist, Justus von Liebig, helped promote a commercially available extract of meat that he claimed would help the body perform extra work. And yes, the secret ingredient in Liebig's "Fleisch Extrakt" was creatine.
Creatine Prolongs Cellular Energy Supply
Nearly all (~95%) of our total body creatine reserve is stored within our muscles. Here, creatine fuels the molecular machinery that generates force (also see Creatine Supplementation Enhances Muscle Anabolics at Several Levels below). And, as a significant portion of our daily creatine requirement is obtained by dietary means (by eating meats and fish, sources of skeletal muscle) dietary supplementation with synthetic creatine salts increases our physical performance.
Expectedly, vegetarians typically possess lower than average muscular creatine levels and consequently, respond quite robustly to creatine supplementation.