LLMs were fantastic for writing a project[0] in a new, niche language[1]. They help write any missing libraries. They help iterate when I get stuck on language aspects. They explain concepts as well as the median SO post, and much better than the posts that don't exist.
[0] https://github.com/TedSinger/chatfile/
[1] https://crystal-lang.org/ - quite pleasant
It does mix up Crystal and Ruby, but the compiler catches it.