This part:
Because flake-parts is using the nix module system. It means that a flake can now impact the host flake output.
This is for scenarios where the host flake gets taken over by one of the input flakes. In a framework-like approach.
I prefer explicit wiring because it’s generally more composable. But I can see the appeal of the above approach if you want to reduce the amount of nix that your users need to know.