Biomechanically it has to do with the muscles in the lower leg. Due to the way the bones are arranged, the muscles in the lower leg move the foot up & down and pivot at the ankles. If you were to pedal with your heel on the pedal, ALL of the muscles in your lower leg goes to flipping your toes up & down in thin air; all that power is wasted. With the ball of the foot on the pedals, you end up harnessing this power into driving the pedals around. The slight ankling motion, while seemingly minor, imparts serious power into your pedal-stroke. Those who play squash or racquetball knows how much more power you get from a tiny flick of the wrist at the last moment of contact.