The systemd unit makes use of the user "signaling", but it is not
created in any way, so the directive is ignored.
By creating a sysusers file it is possible to tell the system to create
a "signaling" user so that the directive is honoured.
For more information, see the sysusers.d manpage, at
https://www.freedesktop.org/software/systemd/man/sysusers.d.html
This is mainly useful on systems running systemd, but the sysusers
concept is implemented also by other projects that don't use systemd,
like opensysusers, originated from Artix Linux.