The rudder is used to keep the nose from pointing high or in other words to keep the tail from sinking. Most non-trainer type airfcraft will need the rudder to keep the axis of the fuselage poniting straight in relationship to the path of flight. As mentioned, it is not to prevent loosing altitude, that is the elevator's function. In a scale aircraft, it take ailerons, rudder & elevator for a coordinated turn.

For those that say the rudder is not for turning. Have you ever flown a 3-channel ship? A rudder that is roll coupled will also bank the aircraft when used. My Sr Tel;emaster had so much roll couple in the rudder, I used full cross control rudder/aileron for knife edge & just released the aileron & keep the (up) rudder in to roll out level.