Because you can turn the Booth up/down to suit your situation... you may need it extra loud to cancel out the main sound system, or you might want to turn it down to speak to someone etc. Being able to listen to the Master is very useful - you can hear precisely what the public can hear. As a DJ, that's important. Unfortunately, many clubs actually have the Master on the mixer turned right down (and the sound system turned right up), rendering the ability to monitor the Master in headphones useless.