External bearings are used to reduce the wear and tear on the servo’s gearbox due to the side loads experienced on the output gear. With modern aircraft getting sleeker and as a result much thinner wings, the servos are too!! this can be a problem for the gears as the servo arms and control horns are reducing in size, increasing the torque loads on the servo arm and as a result putting more strain on the bearing of the output gear/spline.