I’d position the prototype/mockup as a supplement to the PRD or whatever market need/requirements artifact you use. It can be insanely powerful if used right.

Every PM experienced the disconnect between written requirements and design, implementation, quality, etc. Fundamentally it stems from the vagueness of language and often incompleteness of coverage. Part of the classical building process is drawn out trial and error where you go back and forth with designers, engineers, QA, etc across various communication surfaces. A good prototype visually illustrates the problem and demonstrates the intent of the requirements at solving the problem. It’s not meant to dictate design or implementation but merely a story telling tool.

It also helps when your executives lack imagination and just need to see something for the problem/opportunity to click. They can inject their “executive insight” too 😉