This is primarily to ensure that -I nixpkgs=https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz and -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz and -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz behave consistently. It also allows installing packages via "nix-env -iA nixos.<pkg>" rather than "nixos.pkgs.<pkg>". It would be even better to allow "nixpkgs.<pkg>", but that requires a change to nix-channel. Fixes #7659.
