Why Shoot Low?
It's far tougher for a goalie to absorb the puck, or catch it with his glove on a low shot, than it is with a high one, so theres' a better chance of rebounds. As most goals are scored from scrambles in front of the net, more rebounds means more goals.
If your shooting through traffic, like from the point, you have a greater chance of having the puck deflected on net with a low shot, either intentionally from your teammate, or off a random skate or leg from any player.
But there's another reason. Watch and learn ...
More: Shooting Tips