Understanding eval-when-compile

Hello,

I'd like to suggest cover `eval-when-compile` subject in terms of package development. Some topics are bellow:

- When a developer should use `eval-when-compile` despite the fact that at first glance everything works w/o `eval-when-compile`. I.e. some guidance and best practices
- `eval-when-compile` and tests. How it affects and helps. When this may leads to unexpected behavior. Please provide some examples
- Is the presence in a package `defsubst`, `defmacro` `define-inline` the reason to use `eval-when-compile`? Describe why.
- Provide please some anti-patterns or bad practices. When a developer shouldn't use `eval-when-compile` even this works
- `eval-when-compile`, autoloading, package building and distributing. How is this related?
- `eval-and-compile`, `eval-when-compile`, `with-eval-after-load `

Thank you