Services may be in a different mount namespace from systemd for sandboxing or other reasons (also means you have to worry about filesystem permissions I suppose). Passing an fd from the parent (systemd) is a nice direct channel between the processes

But systemd precisely doesn't pass an FD. If it did, you would just need to write() and close().

Yeah I was wrong about that, I confused it with socket-activation passing. The systemd-side socket is available from the process.