I can understand why bringing the mouth forward into a muzzle would help a carnivore (dog, fox, bear) – but why a more or less flat face with a chin?
The chin serves as an attachment point for several facial muscles. I’m not sure I would have included it if I were designing a species, but these things aren’t well discussed in Research and Development, y’know… 
So we can use straws.
Without chins, millions of pillows would be without pillowcases.
You got to hang your teeth on something, which unless you’re thinking of going Remora style, is going to be a jaw. Once you have a jaw, you need some sort of mechanism to keep it from breaking when you wham it into something hard. A chin’ll do for that.
Crocodiles don’t have chins.
Ah, but they do have the snouts, which you’ve already covered. Besides, what are the chances of a tetrapedal croc slipping on a banana, and bashing its chin against a rock? This upright stance we humans affect requires special attention to the strength of the jaw.
Chins aren’t needed for strong primate jaw muscles – all other apes do quite well without them. Gorillas have a high bony riidge atop their skulls for muscle attachment, but chimps and other apes get along fine without any such long-range bases for muscles, and have quite powerful jaws, thank you very much.
I’ve long had a belief that this is another case of sexual signalling and duplication of sex organs.
really.
Humans also have everted lips, which other apes don’t. We favor red lips on women and they often enhance this with lipstick and lip gloss, suggesting the vaginal lips. In a similar way, the human chin suggests the testicles. I find it significant that large chins are not thought attractive in women, but are in men, and that a “cleft” chin (which more closely approximates the double construction of the scrotum) is considered especially attractive in men.
I’m not alone in this. Desmond Morris has suggested it (and I know his name is greeted with derision, but the guy has the appropriate background in primate behavior, and makes some excellent suggestions, along with some absurdities), and one of his illustrated books reproduces a painting which makes the chin < – > scrotum analogy perfectly.
They also have much longer levers working in those long jaws - good ol’ Archimedes, wasn’t it he who said with a lever long enough and somewhere to stand, he could move the Earth?
That being said, IIRC from anatomy class (which I may not), most of the hard working chewing muscles attach in other places along the jaw and the temple and cheekbone - the human replacement for the skull ridge that **CalMeacham **mentions in gorillas. The chin muscles are more for mouth dexterity and talking, which might be why gorillas don’t have chins.
>a “cleft” chin (which more closely approximates the double construction of the scrotum) is considered especially attractive in men
Well, at least noses make more sense now.
As an alternative theory, consider that other apes have more of a snout than we do. Perhaps something about our facial development or diet favored less of a snout, particularly in the mid face, and while both our upper and lower teeth are further back today perhaps the point of the chin has had less evolutionary pressure to move back and so has done so slowly. After all, it’s more in relation to our teeth and maxilla that chins seem to protrude more than in other animals, than it is in relation to our eyes and forheads.
Not to mention that our most immediate ancestors didn’t have chins either. Nor did our cousins, the Neanderthals. The chin appears to be unique to our species, even within the genus Homo.
Okay – so why do we have them?
I read that the prominence of our chin isn’t due to the point of our jaw sticking out more, but that the rest of the jaw has receded compared to our ancestors.
There might not be any reason. It could be the result of a “founder effect” or a genetic side effect of some other trait. Do we even know what genes cause a chin?
There is no factual answer to that question. As others have said, it could be sexual selection, a founder effect, or dumb luck. Not every trait has some purpose-- as long as it doesn’t reduce reproductive success, it won’t necessarily be selected against.
Neanderthals didn’t have chins? What did they have instead?
Their lower jaw slopes away without our pointed, right-angle chin. Apes don’t have chins, either, as I pointed out above. Look at a chimp sometime.
In other words, they have receding chins, I suppose. I would still call that a chin, myself, but I guess you (and maybe the OP) are talking specifically about that thing that’s called the mental protuberance.
Well, if I had to hazard a guess, I would say that the purpose of the chin is to protect your jaw from being shattered when your four-year-old unexpectedly jumps up while you’re hovering over her. That’s what mine is for, anyway.
A lower jaw w/o a chin. Diagran showing Neanderthal vs modern Human skulls.
Dare I say the entire question is rather a mountain out of a molehill (almost literally)? The distinction isn’t hugely pronounced, and may well be a trivial point of genetic history, of course, but even if it had meaning it’s likely not a huge factor.
Does the placement of the chin have some efect on the lips and their muscles? This could have some small implication for speech?