Modern HVAC systems are tightly interconnected to optimize performance and energy use. The hard part is to bridge the designer's understanding of how things will work with the owner's actual operation of the systems. Real life is different than anticipated!
Older HVAC systems, while sometimes simpler, often experience "drift". Sensors have gradually drifted out of calibration, the schedule has drifted to more hours of operation than are needed, and other aspects of operation have changed from the original situation.