The main advantage of volume controlled medical ventilators is guaranteed minute ventilation. This is particularly important in the operating room, where lung compliance may be influenced by the type of surgery involved, and in the ICU or in transit if patient's tidal volumes are not being continuously monitored.
Early intensive care ventilators represented a continuation of operating room techniques, where the patient was heavily sedated and paralyzed until the disease process resolved. The problem, though, was how to get the patient off the medical ventilator before their muscles atrophied. This required some form of patient-ventilator interaction.
 
There is a considerable difference between mandatory and spontaneous breaths. In mandatory ventilation the patient is a passive object receiving gas as determined by the medical ventilator at a set rate and volume (or pressure). A spontaneously breathing (awake) individual demands gas at a flow and rate of their own choosing. Assisted ventilation thus requires a triggering device and a flow of gas to match the patient's peak inspiratory demand (30 to 60 liters per minute). The two methods developed to overcome these problems were assist-control ventilation and intermittent mandatory ventilation.