They guessed at the answer (these are people doing mechanical and electronic designs). They'd test the design, and it would not work. They'd guess again, test, did not work. Quite a bit of time and money would be wasted, whereas some calculus would give them values for their components that would work the first time.

I've also seen people look up formulas in a book, and misapply them with no understanding. The formulas are understandable if you know calculus, but these people would get answers that didn't work and would just be befuddled.