Frequency-Response and Frequency-Domain Models
A major advantage of using frequency response is the ease with which experimental information can be used for design purposes. Raw measurements of the output amplitude and phase of a plant undergoing a sinusoidal input excitation are sufficient to design a suitable feedback control. No intermediate processing of the data (such as finding poles and zeros or determining system matrices) is required to arrive at the system model. The wide availability of computational and system identification software tools has rendered this advantage less important now than it was years ago; however, for relatively simple systems, frequency response is often still the most cost-effective design method. The method is most effective for systems that are stable in open loop but can also be applied to unstable systems. Yet another advantage is that it is the easiest method to use for designing dynamic compensation as seen in the Cross-Reference article:  “Classical Frequency-Domain Design Methods”.