It is a vacuum relief device/valve, for tank protection when draining.
I don’t think I’ve ever used one because we only do gas WHs; I think they are primarily used for electric WHs with plastic tanks, like Rheem Marathon.
We use them on bottom fed water heaters. The heaters do not have dip tubes.
A dip tube has a hole in it approx 6-8” down to break the vacuum. No vacuum breaker required.
Breaking the vacuum prevents the water from being siphoned out of the water heater and even tank collapse or deformation.
So you can burn out elements on electrics and collapse them. On gas you can cause carbon monoxide and tank collapse. Newer gas heaters usually have over heat protection that would shut down in a perfect world.