As we learned in standing, the heel is not a flat surface. It has a pivot point on the medial side which allows for side-to-side rocking. This is for emergency use - for example, when the unevenness of the ground causes us to unexpectedly fall off to the side. In this situation we literally descend onto the lateral side of the heel and toes. This descent signals the brain to recruit leg muscles to stop our lateral topple. Having this safety net to rely on is wonderful. But we do not want to be relying on it habitually. Bracing leg joints is hard work. We want to rely upon the much stronger, medial side of the foot, which will mean less work for large leg and torso muscles, and greater freedom of movement.