The rudder is essential for control of the model n the ground. Taxiing and keeping it on the runway durring take-off.

The rudder is lso essential for knife-edge flight and many aerobatic maneuvers. (knife edge is just flying straight with the airpane in a 90 deg bank)

Generally RC modelers are lazy about using the rudder. this is one reason that when we do a turn and pull the elevator, we tend to also climb. If you use the correct amount of aileron rudder and elevator you will get a circle that does not gain or lose altitude. That is a "coordinated turn."