When does it make sense to create throwaway HTML prototypes for web apps?

I understand how you would go from low (sketches) to high fidelity (clickable interactive) wireframes as prototypes, testing along the way. But if your high fidelity prototypes were actual HTML/CSS in what scenarios would you throw these away in order to start building the site? If your HTML prototypes cover just the MVP (not some future vision past MVP) surely it would be inefficient to throw away the prototype and start building the UI again one story at a time?

I understand throwaway prototypes in other product sense, e.g. plastic moulds of eventual metal products, one-off cars to be tested before mass production, but with HTML you might as well use what you've built already?