They are used on the top of the pool filter tank (and often are referred to as the pressure-relief valve). This valve helps to relieve pressure that builds up inside the filter. Removing this excess pressure is referred to as bleeding the filter.
By opening up, these types of valves are designed to allow excess pressure to escape. At a normal pressure level the valve should close again.
They measure the amount of materials flowing through a system or a part of a system. When open, flow is enabled. When closed, flow is restricted.
The air-relief valve is an integral part of a swimming pool's pumping and filtration system. As the pool's pump sucks out water from the pool, it is forced through a filter. This water is then forced through the air-relief valve.