Typically there are three check valves in the fresh water system; one at the city water inlet (to prevent pump pressure from forcing water out the entry), one at the outlet of the water pump (to prevent city pressure from backing through the pump and filling the water tank and one at the cold inlet to the water heater. It’s there to prevent hot water from migrating back into the cold water system. If a branch cold line, say to the toilet, is plumbed (tee’d) into the cold line too close to the back of the water heater, it could siphon hot water out of the tank and into the toilet when it is flushed. It’s happened before!
Brian, Loretta & Daisy (Golden Retriever)
2008 Holiday Rambler Endeavor 40 PDQ , ISL 400
2014 Ford Explorer toad