Whether you are enhancing your mount setup or buying a replacement part, the counterweight is there to balance weight of the optical tube (OTA) and accessories on the other side of the Right Accession (RA Axis). If you add accessories to your telescope setup (ie camera, guider, etc), you may need to add additional counterweights to off-set the weight.
If the balance is off on one-side or the other, it can do damage to your OTA and setup, the motor gear, and/or may have issues with tracking accuracy. You can add additional counterweights to improve the responsiveness to the system by reducing the mass inertia, though you need to make sure you space out the counterweights along the shaft to reduce damage to your setup and that it is cost-effective and worth it for your telescope setup.
This counterweight comes with a sturdy knurled knob locks to safely hold in place on the counterweight shaft. The counterweight is designed to fit 19mm counterweight shafts