Prior to effective engine control unit computers, vacuum switches were employed to regulate the flow of engine vacuums in automobiles.
For instance, a dual port vacuum switch located in a port on the intake manifold monitored the coolant temperature in the coolant crossover.
It received vacuum (port E on the switch) from the carburetor.
The vacuum flowed through the switch to a vacuum solenoid (such as a heat riser, used to restrict exhaust allowing the engine to heat up faster).
The switch monitored the temperature and when conditions were right it performed its designed function.