Gentle check valves to maintain prime, or stop backflow. An easy check valve is simply a nice way to stop back flow, for example if your filter pump is returning below the water line. If the pump line broke or the pump exploded, without a check valve, the pond could be siphoned off.
Alternatively, check valves can be installed below the pump so that once the pump establishes prime, it can hold prime even when be turned off and opened for service. Too, check valves can keep air from backing up into blower lines in bead filters.
So there's lots of uses for check valves. I bet if you stop and think, there's a use on your pond. These check valves are made of PVC and accomodate the size pipe they're sold to, for example a two inch check valve simply accomodates 2 inch PVC.