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Well, I think I cannot explain any better. The whole point is that the pivot of the rocker arm is plastic and is only fixed in the black half of the extruder body, and thus it is not stiff enough, it bends and causes the guide wheel to be off-center. By adding the bolt, you fix (connect) the free end of the pivot with the clear half of the extruder body. By doing so, you eliminate the wobble and stiffen up the whole thing.