How do you know which control loops are the most important?

PID control loops are implemented to control a process variable (PV) to a setpoint (SP) by manipulation of an output (OP). The relationship between these variables should depend on the key role of the loop or a trade-off between the key roles. With hundreds of control loops active in typical process plants, it is essential for operators to know which ones are the most important and therefore the ones that should get the most attention.