The purpose of the air vent is to let fresh air into the inside of the blower. In the resting position, the vacuum in the boiler draws in fresh air via the ventilation valve through the inner tube and blowpipe.
If the ventilation valve is defective and remains stuck in the closed position, corrosive flue gases can penetrate. Water then often collects inside the blower, which condenses from the residual steam when the blower has finished blowing. In combination with flue gases, severe corrosion damage can occur.
If the ventilation valve is defective and gets stuck in the open state, this can even lead to personal injury if people are in the outlet area of the valve, for example when a runner is inspecting the system.