Comment by _asummers

It had them. But if you did a System.get_env inside the config.exs/dev.exs/prod.exs etc it would be at compile time. Various patterns arose to do that at runtime, by using things like `{:system, "FOO", Integer}` as conventions, but it was always a bit ad hoc. If a lib saw that they would know to punt the actual reading to runtime, which is what many folks did before releases.exs, which runs at RUNTIME, so things like System.get_env work as expected.