My only experience with the climb and descent otto, held the speed or mach, and I set the throttle to whatever I wanted or needed.
Different philosophy in airliners. The aim of climbing is to do it as quickly as possibly to get up into the thin, fuel-efficient air. So the throttles (ATS or not) go to full/climb power and the speed is controlled by the nose position.
but don't know why otto would adjust the throttle(s) as the crew could just set the power to the basic setting from the manaul.
For jets, the best situation is flying at a specified speed eg Long Range Cruise. So the power (ATS or otherwise) is used to control/maintain the speed. Apparently, turboprops use the opposite technique: level off then set cruise power and accept whatever speed you get. Claret will confirm: he has plenty of boat time.
If the policy is to fly the USAF technique of point the jet and use throttle for speed, regardless of the optimum AoA for an approach
Approach speeds are determined to the knot; that gives the optimum AoA. Pitch for attitude and thrust for speed is just another way (IMO more logical) of flying at the correct AoA. After all, that's what the autopilot does on a coupled approach. Maybe when sitting on the back of the drag curve using power to control the descent worked OK; we didn't use that technique in our 190kt-approach land-based delta...