By Stilfehler (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
We are heel walkers. In fact we are very hard on our heels. According to this article our ancestors were more toe walkers.
Barefoot runners are very different than shod runners in the area of the foot that receives the initial impact. The stride impact is directly on the toes with a barefoot runner. YouTube has a video which shows this dramatic shift.
As we age we tend to lose the fatty pad and the heel gets harder and harder. Also the heel is prone to spurs, jagged calcified outcropping from the bone.
In recent years I have focus more and more on the heel. I think the heel as much more sophisticated than I once thought. Because of the heel's role in locomotion I now think of it as a multitude of sensors whose job is to feedback information about the terrain under foot.
It is hard work and can be hard on your thumbs but by working the heel more I find it helps loosen the whole foot. Breaking down the deposits in the heel helps me to reset the this stress level not only in the foot but the rest of the body as well.