Pipework and ductwork systems are typically and traditionally designed for full-load operation. But with the advent of cheaper variable-speed drives (e.g. static frequency inverters)  and the savings that can be made on reasonable size fans and pumps, good control is now required to change circuit flows in a stable manner. One has to understand how the network pressures vary with changes in flow rates and loads, and the relevant tool is network analysis. It also allows complex circuits to be reduced to one equivalent resistance, which gives the system characteristic.