Comment by sshine

I can say what I use flakes for at work:

I have a repository with system configurations for some CI infrastructure: a build server, a test runner.

The test runner can either be generated as an SD-card image using nixos-generators or live-updated using a remote `nixos-rebuild switch`. The OS configuration contains stuff about purposing specific motherboard GPIO pins.

Both systems depend on custom software not in nixpkgs; these are just hosted in private git and have a flake that mainly provide a devShell, but also provide a cross-compiled binary.

Flakes handle all of that in a predictable way: OS images, cross-compiled binaries, devShells, cross-repo linking, convenient hash-pinning.