CHLORINATION - Most pools have an automatic chlorination device like a chlorinator or a salt chlorination generator. The concentrated chlorine level that is discharged from these devices should not flow thru your pool equipment and for this reason the device is installed as the last unit before water is returned to the pool. To prevent water from backing up thru the system after the pump is shut off, a check valve must be installed just before the chlorination device - on the side going to your filter of heater.
SOLAR HEATING - Most solar panels are installed on a house's roof 8' above the pump. To capture the sun's heat efficiently the piping in the solar panels must be full of water continuously. Without a check valve to stop backward flow, the water in the piping would be suck back down when the pump is shut off. The check valve should be installed in the pipe leading up to the solar panels.
SPA - The level water in your spa is often higher than the water level in your pool. When water to the spa is shut off, the spa water level would drain to the level of the pool water. A check valve is installed to maintain the level of the spa water.
WATER FEATURES - Many water features have reservoirs. A check valve is installed in the line to the water feature to keep the reservoir from draining when the pump is shut off.