Well, there are all sorts of things one can use the rudder for.
besides steering and ground handling, you can do knife edges, four point and eight point rolls, a knife edge loop. harriers or hovering need to use some rudder to help keep the plane upright too.
with a plane that had dihedral in the wings, you can turn the plane using rudder, very handy if the aileron servo quits working for some reason.
With a plane without dihedral the rudder pivots or turns the plane sideways, useful for some manuevers to make the plane look better and in knife edges, helps keep the plane level.
On some planes like float planes for example, or a plane with huge landing gear fairings, you need to include a little rudder to get the plane to turn or bank, other wise you have trouble controlling the plane.
When you get into aerobatics you will quickly find using the rudder on the aerobatic maneuvers sure makes them work better and look better.

But one can fly a plane without a rudder, lots of modellers do already. When I fly small two channel 1/2a planes, you only had two channels for control, no throttle, only aileron and elevator or rudder and elevator. I have flown some larger planes two channel as well. Same thing.
Many of the flying wings have no rudder too.