Why is `with rec {}` used instead of `let in`?

[Kamillaova]

1. Why rec?
2. Why not just let in?

[chessai]

1. Why rec?
2. Why not just let in?

1. which place(s)?
2. if it can be changed to let in, it should. in some cases, rec is preferable for sanity.

[Kamillaova]

1. which place(s)?

"import headers" in all files

[chessai]

Would take a PR for this