the rudder is used to control horizontal yaw. in other words it means the rudder is your 3rd dimension. you have climb/dive, roll, and yaw.
Rudders are essential for keep control of your aircraft during abstract manouvers, and especially for gunnery.
imagine you have your crosshair almost on your target, but you are aiming slightly to the left. rather than roll right, pull back on the elevators, then roll left again to move your crosshair onto the target, you can simply apply right rudder to 'yaw' the nose of your aircraft into the correct possition in a matter of nano seconds.
takeoffs and landings, rudder is often needed to counter the torque of more powerfull machines, and also to 'side slip' (flying at an offcenter angle by use of hard rudder and opposite aileron) and lose speed on final approaches... or during a fight to force an enemy to overshoot...or holding course against a strong wind....or....or....well, the list goes on.
during a slow speed flat turn to the left, right rudder should be applied to stabalise the angle of attack and so as not to lose altitude that needn't be lost.
Stalls. you're lost with no rudder in high stall turns and ropes (climbing to the very limit of the engines power before flipping the aircraft 180degs and diving out again, etc)
there are many uses for the rudder on an aircraft, and there are many manouvers and tactics that would be impossible without one.
my advice would be to go offline or in the TA and practice AOA (angle of attack) manipulation. try flying full rudder and opposite ailerons as the most basic exercise, and keeping the aircraft in a steep side slip whilst going in a straight line and not losing or gaining altitude.
know your rudder, love your rudder, run home when your rudder gets shot off.