What possible reason would a president have to move troops around without telling anyone, without it being illegal/dodgy/a bad idea?
I get the impression that it's because getting something through congress means assembling congress which takes time. I'd imagine that this is more of an emergancy power so that if immediate action is required they don't have to go through everything.
It's not like the President doesn't have to face up to congress eventually. If he sent troops in without a good reason he'll get his butt handed too him.
http://www.rockpapersaddam.com/one.html