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’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