What does the Head Angle do?

On a road bike the head angle is about 71 degrees to 73.5 degrees and a 73 degree angle is said to be aggressive while a 71 is relaxed.
They are fine words but what do the words mean actually when the rider is on the bike pushing a hill, turning a bend or cruising on a highway? Just why is a bike with a 73 degree head angle faster, or is it faster, or is it just faster turning a bend going into a bend and coming out. When you have a comfortable rider with a relaxed head angle surely you have a rider with more energy. Why is it the bike that makes the forward motion?
It is always the rider that makes the forward motion and makes the bike go faster. The rider is the engine.