What is the pupose of a check valve on recirculating hot water system?

The check valve is a directional valve. Hot water needs to be piped from the top of the tank to the fixture, because that is where the hottest water in the tank is. If a return line is tied into the bottom of the tank, a directional valve (check valve) should be installed between the heater and the pump. That's what they taught me don't know if it really matter upstream or down stream. If you don't have a check valve the water could flow through the return line to the fixture instead of through the supply pipe and the water at the fixture might not be hot enough.