Prototypes aren't only for UX though, sometimes they're for exploring whether something is technically possible, or what are the unknown unknowns in a particular area.

For example, for personal projects, I've been wondering if it's possible to automatically create RSS feeds for pages that don't have them (yes), what are the challenges when building an archive-style page dumping system (need to dump CSSOM alongside getOuterHTML, remove/rewrite remote content, walk iframes, automate Chrome, scroll to load lazily loaded content, etc.), and if training a model to remove native ads from markdown coming from readability is possible (no, at least not with my current approach, but using the dom might work).