Flakes have a set of default outputs they can generate, like packages, devShells, templates, and NixOS configs. Nix knows how these outputs work and can validate them using `nix flake check`, or list them with `nix flake show`

But what if you want your flake to output a library? A Dockerfile? A Terraform or Kubernetes config? Nix doesn't know about those custom outputs, and doesn't have a way to validate that you're using them correctly.

Now with Flake Schemas, you can provide a schema so that built in tools like `nix flake show` and `nix flake check` can properly validate the new output. This makes it easier for Flake developers to communicate the interface of their custom output to Flake consumers