It's one thing to look at the source, it's another thing to know that you have to look inside `pkgs/build-support/trivial-builders.nix` to find the documentation. One of the main advantages of reference documentation is a central place to look up what things do.
`writeScriptBin` is the second topmost link on Google search that leads directly to that file in a central repository of nixpkgs. Unique function names have the advantage of being transparent references that you can find either via google, or github search, or grep on your local checkout of nixpkgs.
