As many golfers are very well aware, the harder you go after a shot does not mean the farther the ball will go. The reason why this statement is true is because the most important thing that needs to happen to hit a golf ball solidly is the golfer must hit the back of it.
Now that we agree on that, how is a golfer supposed to hit the back of a golf ball when all of their weight is going out towards the ball? If the golfer's knees start to crowd the space where the golfer needs to drop the club to get the club into the "slot" how then is the golfer going to hit the back of the ball. It cannot be done. For the golfer to hit the back of the golf ball squarely, their weight must be moving parallel to the target. Two things will happen when the golfer's weight moves properly parallel to the target. 1.) Space that was once not available for the golfer to have a good angle of approach into impact is now available. 2.) Power created by the lower body is now working out towards the intended target, instead of being wasted out towards the toes.