As SteveO suggested, it is clear from the exploded diagram that a bearing is part of each Tail Gear Case Plate, and the Tail Output Shaft Stopper is locked to the output shaft using the set screws, which sit in the D grooves in the shaft.
The Washer and Poly Slider precisely set the distance between the left bearing and the mesh gear (actually a mitre gear), while stopping the shaft from sliding up/left on the exploded diagram and disengaging the leader and follower.
The purpose of the Tail Output Shaft Stopper is to stop the shaft being pulled in the opposite direction, which is the direction the blades are going to be causing the shaft to be pulled when rotating.
With the stopper in place, the sideways force would be applied from the shaft to the bearing, through the Stopper, with very little force applied to the mesh gear.
Without the Tail Output Shaft Stopper all of the sideways force would be applied via the mesh gear, potentially wearing it out sooner, and possibly even causing it to bind and stop rotating altogether.
This sort of arrangement is fairly common in robotics application.