http://www.npr.org/2016/01/29/464893281/why-do-humans-have-c...
In short. It's unlikely to be for protection, or for sexual selection, or to aid in speech. Go to the interview detailed explanations.
Most of the comments here offer reasons that are even mentioned in the articles as plausible.
Others have suggested that the chin is an adaptation for chinwags: It resists the forces we create when speaking. After all, speech is certainly a feature that separates us from other living animals. But there's no good evidence that the tongue exerts substantial enough forces to warrant a thick chunk of reinforcing bone.
That's not my first thought at all. I think it has absolutely to do with speech, but not for that reason. I think it's to create more space for the tongue to move around and make different sounds.
Just another SWAG, of course, but I'm surprised not to see this idea floated.