A prototype answers one question cheaply. Not ten questions — one. The teams who waste money on prototypes are the ones who treat them as a rough draft of the whole product; the teams who benefit treat them as an experiment with a pass/fail answer.
Prototype when the question is expensive to get wrong: will farmers carry this device every day? Will customers trust a face-verification step? Will the sensor survive the rain? Those questions cost little to test and a fortune to guess.
Skip the prototype when the question is already answered — a marketing site, a CRUD platform, anything where the risk is execution rather than discovery. There, the fastest path is simply to build it well.
Our rule from three years of field work: if a wrong assumption would cost more than the prototype, build the prototype. That is the whole decision.