Humans have strong teeth and impressive biting strength for their size. The front teeth in particular are designed to tear through flesh. Biting causes about the worst flesh damage unarmed combatants can do to one another: pretty much every other strike involves either blunt force trauma or relatively slow strangulation.
Obviously it's got a lot of drawbacks: disease risks (for both sides), an extremely limited reach as well as being a very committed attack: the attacker can't defend their head while using it. It's also, technically speaking, extremely sickening. Most of the time, biting is a last resort attack.
In the martial arts, biting is a documented method of escaping a hold while one's limbs are pinned. It is taught in only the most pragmatically brutal disciplines: actually using it in anything other than a real world life and death situation is near unheard of. Proper utilization requires racking off a piece of flesh to convince even the most determined of attackers to let go.
However, it does have a very large psychological impact. Having someone try to bite you, or even lunge at you snapping, is far more disturbing than seeing a punch coming, due to the fact that it taps into a far more primal response. It's often used as a way of getting someone to rear back, or flinch, so that you can get a hand free or get enough space to use your head. Adding to that, it hurts a lot.
There is also a considerable amount of bacteria in the human mouth - as many as 190 different species, and a hundred million organisms per milliliter of saliva. Breaking the skin with a bite leads to infection ten to fifteen percent of the time (usually a severe staph infection at that, as most humans have Staphylococcus aureus comfortably living on their teeth), and hepatitis, herpes, syphilis, tuberculosis, and tetanus have all been documented as being transmissible through human bite injuries.
Finally, "fight bites" occur when the fist connects with a mouth not currently being used for biting and is cut by the teeth. Along with the fact that human skulls are hard and likely to break fingers, this is a reason why barehanded fists to the head are better used with caution.
