For the water which is in compressible, to drain out of the WH, you need to replace the water with air. How does the air get into the WH? If the check valve isn't there and you think the water is coming out of that inlet, where is it going (through the plumbing) and what's forcing it to flow?
Ted
Air will get into the tank when a faucet is opened.
My previous boat's water heater had a check valve built into the inlet adapter. I don't know about my current boat, where I live now I don't have to drain it.
Gravity causes water to flow downhill. That could happen in some installations.
The important thing is to follow the instructions from the manufacturer of the water heater. We don't need to ask why or second guess. RTFM!
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