Ancestry of this object goes back to the first parametric file set for the Mendel, and in particular the Z pulley.
OK if we lived in a perfect world, this thing shoudn't be needed at all. Its intended purpose is to deal with the consequences of the flexing of the motor mounts related to belt tension, above all on the Simplified Prusa Mendel's X axis.
Because of the flexion of the mounts, we get a lack of parallelism of the axes, and the belt wanders on the pulley and gets grossly out of alignment with the carriage.
But why are the motors mounts flexing?
Because plastic-saving design has got the pieces quite thin. Actually excessively thin for their intended function.
So all of this could be easily solved by putting back plastic where it is structurally needed! (and we'll save quite some time and zip ties). Designers, wake up! You have gone too far with some reductions in plastic! We need significantly stiffer mounts for the Y and X motors! (I'll try to help with design, but this will take some time).
Checking old files: Selis Mendel's massive Y bracket was 17.5 mm thick!
So on the interim, to help the belt keep straight, we have this cap (and crummy zip ties and...)