A light can be overlooked(how many vehicles have you seen going down the road with the blinker on) The buzzer GETS YOUR ATTENTION that you have a serious problem with the vehicle.
My work vehicle has both- light and buzzer on the air, just light on everything else. I have had the engine go into limphome mode due to loss of coolant- If I had seen the stupid light, I would have stopped sooner, and I am a professional driver(at least I get paid to hold the steering wheel)
The buzzer and light comes on at 65 PSI on work truck. The trl valve comes out at 45-50 psi, and parking valve at 30-35. So if the buzzer comes on, I start looking for a landing zone very quickly.