After you’ve learned the things that can be learned, these become completely useless, yet, we still can find random inventions, which we break down for scrap (which is more valuable than a one time use thing, assuming it’s not a ‘break down only’ version), or when sifting scrap to get random parts/toys.
With these being constantly generated even after learning all the recipes, let us do something with them, like using them in Invent to get KP, maybe needing five or ten of them per KP so it’s not a trivial amount but not so rediculously high to feel like a gag. Heck, being able to sell them to a special vendor for things (like say Crowd Pummeler 2 parts) would be just as fine imo.
The point is - why does engineering, which already struggles with being relevant outside making a handful of tools and toys, have a ‘limited number of uses per character’ resource that is endlessly generated?