I would add on top of all these answers that medically induced comas have started being used on rare occasion as a treatment for rabies. The disease gradually causes massive firing of the victim's nervous system that gets worse over a period of days as the virus spreads toward the brain. If they're conscious, the person dies from seizures, disruption of breathing, and heart failure.
If they're in a coma, the nervous system is able to shut down. The virus runs its course and is soon eliminated by the immune system, and then they are able to end the coma by withdrawing the anesthetics.
The person is given drugs that heavily sedate him or her and paralyzes all muscle activity (even breathing). The patient is put on a mechanical ventilator and is given nutrition via IV or feeding tube. This is done for patients who need to recover from brain injuries that often cause a patient to be disoriented, upset or even combative. It can also be used for patients who would otherwise be in great pain or discomfort during the recovery process. The patient is very slowly weaned off the medications and the mechanical ventilator until the doctors are confident that the person is able to breathe on there own
Medically induced comas are just like it says. Comas that are induced with medicines. A frequent drug used is called propofol. It's great for knocking someone out and is quickly metabolized. So when you stop it the person wakes up fairly quickly. This is a great drug to use for patients with neuro problems bc you can wake them up frequently for assessments.
Now why are medically induced comas used? They are used for a variety of things and not just head trauma. Respiratory failure is probably a more common use. When someone is in respiratory failure or their airway is at risk of being compromised the patient is intubated. In order to be intubated you have to be sedated or essentially in a medically induced coma. Other uses would be for someone who is having seizures or in status epilepticus.
Now for head trauma the biggest fear is an increase in intercranial pressure which can lead to neuro deficits and death. Putting someone in a coma can decrease that. This is essentially bc you are decreasing their exertion and stress levels. However it is important to note that when in an induced coma they need to be mechanically ventilated. Mechanical ventilation can increase intercranial pressure. This is through PEEP or positive end expiratory pressure. In vent settings this can help improve oxygenation but will impede venous flow and lead to an increase in intercranial pressure. So its fairly complicated to manage without a strong understanding of the physiology.
As far as waking up that depends on what drug was used and the strength of the patient. Some patients may have weak lungs to begin with and weaning them off of vent support is very difficult. The longer someone is in the hospital bed the weaker they are and the harder it is to take them off support.
I had a friend who caught swine flu a few years ago in conjunction with pneumonia, and putting him into a medically induced coma saved his life. By putting his brain to sleep, it allowed the anti-viral medication to work more effectively in his head and then they slowly brought him out of it once the infection has cleared from his body.
A good, over-simplified answer for this would be that the coma causes the brain to focus on less activities such as breathing, being alert, and simply living.
Medically induced comas are a mix of drugs that cause all these levels to be non existent and simply put the brain on "hold" to allow it to recover.
Additional question to OP: What would be a reason/reasons someone would not regain consciousness? Or all brain activity would be gone?