Condenser fans are found in most refrigeration systems. The fan moves air over the condenser coils to remove heat. In many systems, the fan is running constantly running and often on just a single speed. A fan controller would slow down, or stop the fan when the compressor is not working, significantly reducing energy usage and noise.
Evaporator fans are mounted inside the cooled space and will move air over the cooling coils. Ice building up on the coils would cause a big reduction in efficiency, but the evaporator fan helps keeping the coils dry, moving the cooled air out in the cooled space. Since the motor is often mounted inside the cooled space, the motor will generate heat when it is running, so it may be desirable to only run the evaporator fan when needed.