Used to regulate water flow through a pipe. Allows for complete flow when fully open.
Use to provide flow in one direction only through a pipe. When installed before an external filter it keeps the dirty water in the filter from flowing back into the pond if power is lost. Also to keep an external pump primed if power is lost.
A gate valve is also called a knife valve. They are easier to use than a ball valve for fully opened or closed operation. Not for regulating flow.