it's an interesting point. is it worthwhile to struggle through an incidental task that has been solved before? we all stand on the shoulders of giants

I think in most cases, understanding is the point. we don't expect students to derive general relativity before doing astrophysics. re-invention is only a tool for understanding

“Understanding” without being able to use that knowledge for anything isn’t useful for getting stuff done.

The flip side is even more interesting. There’s a great number of electrical engineers or even with significant physics backgrounds who don’t really understand how electricity actually works, but they can still solve useful problems. By understanding I mean they can describe what underlying physical phenomena reactance represents etc.

Small counterpoint to your analogy, as someone who studied astrophysics: I actually did have a requirement to understand general relativity! Deriving all of it independently from scratch wasn't something we did, but there _were_ derivations involved. And it was definitely worth working through -- it _is_ a good tool for understanding. (I've long since left the field, but I don't regret the work I did.)