The secondary adjustment is provided to adjust belt deflection as belts wear and are replaced on polaris sleds . The OP’s secondary as shown is adjusted for a belt that is near the end of it’s life, or due to manufacturing variations the belt that was replaced was alittle bit long. 

polaris provided this easy way of adjustment so that a person didn’t have to jack around with loosening engine mounts and torque arms in a attempt to slide the engine around.

In no more than two minutes the belt deflection can be adjusted with no more than a 10mm wrench.