This Vixen 8.2 lb (3.7 kg) SX Counterweight is used to add stability to German-type equatorial mounts.  While your standard counterweight might be sufficient for observing at 45° of declination, when you're at or near zenith it might be a different story.  The same is true as you add various accessories such as cameras and off axis guiders; these all add to the weight upon the scope's mount, which in turn necessitates additional counterweights.  It is a good idea to have enough counterweights on the mount to handle the scope easily at zenith, and then when you're at more middle declinations you simply move the weights up towards the neck of the mount.