A gas control valve can be found on any appliance that utilizes gas. These valves are vital for the operation and safety of any gas appliance. The valve does exactly what it sounds like, it controls the amount of gas flowing to create a steady flow without allowing any gas to leak.
The most common gas appliance found in US homes is the hot water heater, although the valve serves the same purpose for heating systems and stoves as well. If you need to know the specifics, here is a brief overview detailing how the control valve functions.
A gas burner sits underneath the appliance and heats by burning the gas that is supplied. The control valve regulates the gas that is supplied to the unit.
Another function of the control valve is to sense whether the thermocouple is being heated or not. As the gas is burned by the pilot light, this warms the thermocouple. If the right temperature is not reached, the valve will cut off the supply. This prevents potentially deadly gas from escaping into the environment and possibly being an explosion hazard.