Fill valves are essential components in various plumbing systems, primarily used in toilets and tanks. Their main function is to control the flow of water into a tank, ensuring it fills to the appropriate level after a flush. When the water level drops, the fill valve opens, allowing water to flow in until the desired level is reached, at which point it closes automatically. These valves come in different types, including ballcock and floatless designs, each with unique mechanisms for regulating water flow. Properly functioning fill valves are crucial for maintaining efficient water use and preventing leaks or overflows in plumbing systems.