One class of use cases not covered in the blog post is that a variable appearing as a function's argument can only be a bound occurrence, a use of some name already in scope. A variable appearing as a macro's argument can be the binding occurrence of that name. This is why (using Racket examples) match, define-struct, for/list, and such things aren't functions.

[1]: https://docs.racket-lang.org/reference/match.html?q=match#%2...

[2]: https://docs.racket-lang.org/reference/define-struct.html?q=...

[3]: https://docs.racket-lang.org/reference/for.html?q=for%2Flist...