This graphic illustrates a conventional control valve, and a delta-p valve installed in the same system on the return side of two coils.

The variation in supply and return pressures across the valves is the same. Note that the flow rate through the pressure-dependent valve changes as the differential pressure changes. In contrast, flow through the delta-p valve remains constant. Constant flow precisely matched to the heating or cooling load, optimizes the heat transfer, minimizes circulated water, and ensures that high delta-t is achieved at all load conditions.

The conventional valve is simply not able to keep the flow matched to the load. Flows change as pressure changes, and it acts like an on/off valve at low flow.