It’s called an improper list and is useful in IO, usually used to build iolists ["like " | "this"].
Might be somewhat relevant:
https://www.evanmiller.org/elixir-ram-and-the-template-of-doom.html
https://www.bignerdranch.com/blog/elixir-and-io-lists-part-1-building-output-efficiently/
