Linearization involves creating a linear approximation of a nonlinear system that is valid in a small region around the operating or trim point, a steady-state condition in which all model states are constant. Linearization is needed to design a control system using classical design techniques, such as Bode plot and root locus design. Linearization also lets you analyze system behavior, such as system stability, disturbance rejection, and reference tracking.
An alternative to linearization is feeding input signals through the model and calculating frequency response from the simulation output and input. You can use frequency response estimation when the model cannot be linearized because of event-based dynamics, such as those associated with pulse-width modulation and Stateflow® diagrams.
For more information on linearizing Simulink models, see Simulink Control Design™. It also provides functions for calculating frequency response without making changes to the model.