Sberry... I have a couple of 100' portable hand carry air hose reels (by Rapid Reel, with self supporting stands). I can have both connected to a semi portable compressor at once, providing two separate circuits of air, often regulated to different pressures, deployed simultaneously.
Sometimes, I want to disconnect one of the hose reels, and daisy chain it to the end of the other, to achieve 200 feet of reach without moving the compressor, since extending the air hose adds tank volume, and extending the power cord adds electrical heat, load, and loss of power.
When I disconnect a 100 foot air hose, there is a lot of air in that long skinny flexible air 'tank'. I don't mind dumping that air, but I'd rather not waste the air already in the steel air receiver, and I'd rather not waste the electricity pumping the entire air receiver back up again after the disconnect. And it sometimes takes a bit of wincing to use the coupler alone to release a 100 foot air line at 90 psi. So a safety ball valve that dumps the air downstream (hence draining the hose I wish to decouple from compressor, without draining the compressor tank) can be quite useful.