IF you don't bypass the water heater tank when using air, then you've got to pressurize the entire volume of the water heater tank along with all the plumbing lines. I suppose that's not a problem if you have a large air compressor, but I've got a tiny little one with a 1 gallon tank. It would take a long time for that little compressor to get everything including the water heater tank pressurized to 20+ psi or whatever air pressure you like to use for the blow out. So I always bypass the water heater tank, drain it, and leave the plug out for the winter.