airline cabins are pressurized at a max of 8000 feet
fighters are pressurized at a differential pressure schedule. At 50,000 feet the cockpit is at 25,000 feet.
at 30,000 feet the cockpit is about 12,000 feet
above 10,000 feet cockpit pressure the O2 regulator automatically starts supplementing cockpit air with oxygen through the mask and continues doing so until it is 100%.
Combat edge increases that and supplies positive pressure 100% O2 under G.
So, the mask is not a backup. It is absolutely required for life support.
It also holds the microphone for comm, So you need it all the time. Very rarely does anybody ever drop the mask, to eat, drink on long missions, and then only to take a bite, then chew with the mask on. but that is about it. Fighter pilots dropping the mask like in the movies simply does not happen.