Prototypes are arguments, not artifacts
A good prototype is not a smaller version of the product. It is the fastest way to find out whether the product is worth building at all.
Teams often treat a prototype as a rough draft of the real thing, a partial product that will slowly be filled in. That framing wastes the most valuable thing a prototype offers: the chance to be wrong cheaply.
A spec is a guess
Specifications describe what you think you want before you have seen it. They read with confidence and ship with surprises. A prototype turns that guess into something you can hold, click, and react to, which is the only reliable way to discover what you actually need.
Build the argument, then the product
We think of a prototype as an argument for a particular direction. It says: here is how this should work, here is what it would feel like, here is the part we are least sure about. Put it in front of customers, stakeholders, or investors, and let them attack it. The prototype absorbs the criticism so the production build does not have to.
The result is not a head start on code. It is a clear answer to a more important question: should we build this at all, and if so, which version? That answer is worth more than any artifact.