This is a good kuchchan, and there doesnt appear to be specific use of the chin.
It could be that the size of our jaw (homba) reduced, as we lost a few teeth and our face gained a "softer" look when we learned to cook with the invention of fire, leaving the "chin" protruding. We didnt need the tough jaw and the strong nut cracker teeth anymore as our food became softer to munch with cooking. The chin could be something that is left after the jaw line receded. Also maybe sexual preference, where person with a chin was deemed more handsome and gained more partners causing us develop a sharper chin over the many thousands of years of evolution.