It teaches you the language of systems theory. Once you master the concepts in Desoer and Kuh, you realize that an RLC circuit, a mechanical spring-mass-damper system, and an economic model are all mathematically identical—they are all just differential equations dressed in different clothes.
For those unfamiliar, Ernest S. Kuh was a pioneer in network theory and computer-aided design at UC Berkeley. Desoer (also UC Berkeley) was a giant in nonlinear circuits. Together, they wrote a book that assumes you want to think like an engineer, not just solve for current ( I ).
For many years, the original hardcovers were difficult to find or prohibitively expensive. This led to a surge in students looking for digital copies to supplement their coursework.