Why is there a low water cutoff valve in a boiler?

The low water cutoff valve in a boiler is a safety feature. If the water level becomes too low, the valve allows more water in to prevent damage from the boiler overheating. If the low water condition continues, the low water cutoff valve will close off the fuel supply and trigger a low-water alarm. In some configurations, the makeup water valve and the fuel supply cutoff are combined into a single unit, and in some boilers they are two separate components.