If you are talking about a pneumatic solenoid valve the function of it is to control the air in and out flow in a pneumatic actuator.

dfrere

to amplify the input pneumatic signal

One of the application of the control valve with positioner is that it is used to control the opening and closing of the actuator based on electric or pneumatic signals. The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators.

The speed of a pneumatic motor is controlled by regulating the flow of intake air with a proportional control valve

Paul Berbakov