It doesn't really add any performance to the gun. It make it so the gun is closer to you. The Dye on/off has a little screw on it and when you screw it in it puts air into your gun. Screw it out and it turns the air off but there will still be one or two shots in your gun (maybe none) depending on what gun you have.
The on/off also makes it easier to unscrew the air bottle as you have released the pressure beofre you start taking off the air bottle.