You go outside to wash your car, connect the hose to the hose bib, and crack open the valve just a bit. A thin stream of water exits the hose. You open the valve a bit more, and more water flows. After about a half-turn of the handle, you have about as much water coming out of the hose as you’re going to get.
But wait! You turn the handle another full rotation, but no increase in water flow is available! What happened?
What is happening is that at full flow (one-half turn of the handle, in this case) the pressure drop across the valve is about the same as the pressure drop through the hose. The valve can move much more water than the hose can.
Now imagine that you are sizing modulating control valves for heating & cooling coils. The contractor wants to install line-size valves without those pesky reducers, and the engineer wants plenty of safety factor in heating or cooling capacity. If I size valves this way, I will never get good control from my valves. Only a fraction of the valve stroke (on globe valves) or valve rotation (on characterized ball valves) will produce full flow through the coil. The remainder of the valve stroke will be wasted, just like the extra handle turn on your hose bib. In addition, the programmers and technicians have to tune their control loops to better utilize the reduced useful stroke. And then, the owner wonders why he is at full heat with his valves only 25% open; or why the system struggles to control at light loads.
By properly sizing a modulating control valve, the customer receives a better operating HVAC system. Although nearly impossible to get perfection, 0% to 100% valve position has a much better chance of producing 0% to 100% heat. The control system is less likely to hunt, and control in lighter loads becomes much more manageable.