Why root locus method is an important tool in control system design?

The root locus shows how your poles and zeros move in the S-plane while you vary the proportional value ';K'; (usually).





the pole positions tell you about stability and settling time, you don't want them on the right side for example.





When you design your system, you leave K as a tuning parameter, the root locus tells you how changing K affects the system.





There is no general rule, it depends on what you need for each system, and you tune K to be what you want. For one example, in a system that might face temperature variations where some elements will affect the proportional value, or some other source of error for that matter, you want to place your roots and adjust K so that it's not at the edge of instability for example.