Fresh Legs

One often hears in football matches of a player being brought on because of his fresh legs. I have to say this has always baffled me. Are any of my countless illlustrious readers blessed with a pair of these invaluable commodites- the famous "fresh legs"?
An afterthought strikes me: if these fresh legs possessed players are so useful to a team's chances of winning, why are they not played from the start of these matches.