> What's the advantage to ...

For me personally, I'd say "not much". I'm used to using the latter form, and it's fine. But I will say that now that I know about this, I see using it in scenarios where I'm running a lot of systemctl commands over and over again in close proximity and have the "muscle memory" of typing "systemctl" more in mind.

For example, when working with a new service that isn't quite working right yet, and doing many iterations of:

    $> systemctl start something.service
    $> systemctl status something.service
    $> journalctl -xeu something.service
    $> emacs whatever
    $> systemctl start something.service
    $> systemctl status something.service
    lather, rinse, repeat

Especially if I'm testing on my laptop AND a remote deployment, I think it's easier from a cognitive viewpoint to always "think systemctl" instead of having to "think systemctl" sometimes and "think ssh systemctl" in others.

To be fair though, it's all a pretty minor point. But I do think it's cool that systemctl has that option. shrug