When the car's empty, the springs are supposed to support the weight of the car, keeping it level. However, as you ad weight the load-leveling suspension's triggered to compensate, then the struts help to support the weight.
Regardless, when your car's running the car should be level...either supported by the springs alone or supplemented by the load-leveling shocks. If the rear end's always low, then you would appear to have worn springs in addition to load-leveling system issues.
Hope this helps.
Best regards, Michael