The air gap is used to protect against back syphonage as normally you do not want a physical connection of a drain pipe to anything that comes into contact with a potable water supply, this is also why all faucets and water supply piping have an air break 2 times the effective opening.
OF COURSE Wallford P&H will be glad to explain how the code officials derived at these sizing of the various air spaces.
Actually Wallford would be more then happy to explain why some codes don’t require a vent what so ever on this type of fixture.
BUT I am still waiting from W P&H to explain about the “R ” stamp requirements in HEATING