Not required on a pony (AKA tiny) turbo like a T3/4 (as the wastegate is fully closed at high rpm (if a strong enough actuator is used) and the engine is consuming all the air the turbo can provide, hence the boost tail off). However, on a bigger turbo that can actually provide more air than the engine can consume, the air injectors are used to accurately bleed off air to operate the wastegate capsule (actuator) to modulate the wastegate to obtain the correct amount of boost (as programmed into the ECU).

At this level, the amal valve isn't really accurate enough, as the boost pressure can force open the spring in the amal valve, allowing boost to climb.