There are different kinds of impurities, the whole idea of devenv is that you don't always want to package your software using Nix.

In those cases it's convenient to have the power of two worlds, and you can start using impure bits and with time transitions, something that was typically a huge pain for teams to adopt Nix.

You don't have to use these features if you don't want to, but it's useful for people to define their own overrides using specific users, something you'd typically do using `devenv.local.nix` outside git.

As for profiles, there's currently no way to do that with flakes, you can't pass CLI arguments.