Wastegate DC and Turbo Dynamics adjustment - WRX COBB reflash

Under practical reflash tuning, I am watching the WRX/COBB reflash portion under optimizing the tune and I am just wondering how wastegate duty cycle adjustments equate to the fairly large horsepower and torque gains across the powerband? Does it have something to do with less stress on forcing boost levels higher, faster and allowing the car to work where it is comfortable?

Any insight would be helpful