The Fokker Eindecker's (and those similar) comma-shaped rudders and aerodynamically balanced elevators were free to float to whereever the airload would take them, making the planes physically exhausting to fly. These surfaces had to held in position, as there was no way to trim them.
Adding the fixed vertical and horizontal surfaces relieved a large part of the pilot's load, the surfaces streamlining behind the fixed surface restricted their motions with gusts and off-center air loads.