These are a common water valve used on many pools and spas. They come in various sizes and provide complete water shut-off. They are cheap for a reason, and are not recommended.
We do not recommend these valves. The handles are not very well UV-stabilized and crack over time. The internal plastic seals also swell and crack over time, which make the handles almost impossible to use. We've actually had an incident where a handle broke while turning, causing it to slice into the palm.
The valve handles get very tight and can snap off over time. We ourselves do not use them on our own installations. They are a cheap, budget valve for a reason. We don't recommend these type of valves.
Better valves are listed here: Jandy Valves