It allows your side weights to stabilize yaw and pitch without affecting the overall front and back weight balance of the bow.
imagine your bow is swing left and right parallel to the ground, to correct it you have too options:
1. you add 3 oz to the front, but your bow is now head heavy, to bring it back to the way it feels before, you now have to add 3oz to the back. our bow is 6 oz heavier.
2. because our bow isn't rolling too much, we simply angle the rear bar downwards by ten degrees, our bow will still be a bit head heavy because we shortened the rear weights, but only by about 17% or so. so we will need to add at most 3 oz to the front to to obtain the same feel. but most importantly, our bow is only 3 oz heavier.
Whether you should do it or not is a whole other topic to consider, but that's mostly why the pros to do it. They trade off roll stabilization for a more customize able weight distribution with less overall dead weight on the bow.