A friend of mine's dad had the same thing happen to him in ICU after a heavy car accident.
The explanation was that the patient has a more speedier recovery as the patient is not actively feeling the pain and can not think about what has happened or what is going to happen, which of course creates a lot of stress. By inducing the coma the body and brain are set for full recovery mode (unconsciously) and hence do not have any "side effect" disturbances create obstacles during the recovery.