I suppose I'm thinking specifically about head injuries, though this question applies to any injury that causes unconsciousness for, say, a few minutes. If you would revive so soon, why would you fall unconscious in the first place? What exactly is happening physiologically during brief unconsciousness? Is it a sort of healing process? Once you're unconscious in cases like this, what causes revival? Also, is such unconsciousness on the same spectrum as coma, physiologically, or do they have different causes and/or mechanisms?
Loss of consciousness is usually caused by a lack of oxygen to the brain - in the case of injury it could be directly cause by the injury or it may be because they are shocked or scared - this is called vasovagal syncope. Basically, severe emotional stress leads to a burst of activity in particular nerves that lead to reduced heart rate and dilation of vessels throughout the body, reducing blood pressure suddenly. The effect is temporarily reduced blood flow to the brain, which causes temporary loss of consciousness.
There is a powerful theoretical consideration that a loss of consciousness is an internal protective mechanism.
Because the loss of consciousness is a significant threat in a hostile external environment, it is only when the nervous system itself is severely threatened internally that consciousness becomes compromised.
Further, loss of consciousness can be protective in the face of physical and emotional trauma, against shock and by inducing the 'ragdoll' effect.