Here's a typical classic anticipation and overshoot.Moving away from the start pose and then going past the end pose - then settling into the final pose gives the action more space. More space for the same amount of frames = a stronger accent. If the animator had just inbetweened the start and stop pose, the action would have had no impact. It'd be mushy.
http://www.cartoonthrills.org/blog/01Principles/antics/WoodySolidanticMad.mov
Here it is with every frame: