The hookah harness or a BCD keeps the air hose from getting in the diver's way when working underwater. Hookah harnesses are sometimes integrated with a weight belt, to counteract your body's natural buoyancy and make depth control easier.
The hookah harness typically has a backplate which is automatically positioned over the centre of the diver's back when the harness is worn. The backplate is often used to hold a bailout or emergency air source system. The backplate holds a check valve, which acts as a junction point for the air hose and the regulator. Since the air hose terminates at the diver's back, it prevents potential entanglements around the diver's body.
The regulator intake hose that attaches to the check valve prevents any pulling motion from the regulator while working underwater. For example, if a diver were moving around underwater and inadvertently came to the end of the air hose, the harness would absorb the shock and the regulator end would not be jerked from the diver's mouth.
The check valve that is found on the backplate performs a third very vital function. It acts as a safety gate by shutting down the air system, allowing the air to travel in only in one direction. Should a burst or leak occur in the airline somewhere between the output of the compressor and the input of the check valve, it could prevent a vacuum occurring in the mouthpiece of the regulator or cause a diver to breathe in a large amount of water that could cause panic.
Never, ever, dive without a hookah harness or BCD and a check valve!
Please read Introduction to Hookah Diving for more information about things you need to consider with a hookah diving setup.