In design an engineer rarely starts with a blank sheet of paper. Designs are usually the result of the improvement of an existing system, the innovative combination of existing systems, or the application of new technology or new knowledge to an existing system.
In all this, understanding what exists is paramount and modeling is essential to that understanding. The purpose of modeling is insight! Also, once a concept has been developed in the conceptual phase of design, it is evaluated through modeling the physical system, sensors, actuators, and controls – not by building and testing – all integrated into the design concept.