What is the need for air lock relay in control valve

Explain why air lock relay is used with control valve

To hold the control valve as per command while failure of electrical or pneumatic signals. air Lock is the 3rd position of the control valve in which the fail-safe position would be neither open nor close and valve position would be sustained as it is as operating position. so the air lock relay sustains the valve at the operating position. for example, if valve is operating at 60% so it would be at same position in case of electrical or pneumatic signal failure or in case of any fault

An airlock relay is used when it is required that the air pressure to the actuator be automatically held on the same level unchanged from the pressure prior to pressure change in the air source.