I read somewhere sometime ago that the theory that makes the most sense is that we evolved chins to take a punch, which animals besides our immediate evolutionary relatives do not do.
So we evolved chins as an evolutionary advantage over our immediate evolutionary relatives who would logically be competing for the same resources.
I read something at some point about how our fists seemed to have evolved or at least adapted to be well suited to delivering a punch. Many people do not use proper hand forms for it, but I suppose it's a learned skill if not at least through trial and error.
Great, now I have a scenario in my head where one early human delivers a punch and dies of having not the right hands and the other dies because no chin.
Also: Imagine our fists if there had been more habsburgs.
I'm a little shakey on the details but I think it has more to do with the extra bone mass in relation to the way the jaw functions. It acts more like a shock absorbing bumper than a lever.
Other animals that get punched there tend to take that blow more to the throat, which is a bit more problatic