Theres a couple of things that rudder are used for. I know in a full scale plane it is used for coordinating turns so that the nose of the aircraft is moving in the same direction as the turn. That way you arent sliding through the air in a messy looking turn. It requires right rudder when your in right bank, and vise versa. Another thing that rudder is used for is on crosswind landings. In a crosswing you need to apply alot of rudder and counter with bank in order to come in level. There is also some acrobatic manuvers that rudder is needed for.