Why do we have a gag reflex? what controls it?

Gag reflex: Contraction of the back of the throat is triggered by an object touching the roof of the mouth & back of tongue is a protective mechanism to help prevent choking. The reflex action involves two nerves, one is sensory from the throat (Glossopharyngeal nerve) to the brain, and the other one the motor nerve (Vagus nerve) from the brain to the throat.