Sort of get it now. For eg, if i were to set the high boost at 12psi on the controller, what it does is to trick the actuator into keeping closed until boost pressure reaches 12psi and then the controller will do as what paulr33 describes as "tell the actuator to open" and the actuator will open up the wastegate to regulate the pressure. Or else, without the controller, the actuator will always open up at 7psi and boost pressure can't go up anymore. The controller does the same thing for any boost setting.
Is my understanding right? Please correct me if wrong anywhere. Also, if the actuator will always open up at 7psi without the use of a controller, then why do most of you guys use an aftermarket ecu other than rearranging the fuel map for tuning?