If I want to use a tmpfs for /tmp, I can just enable the systemd tmp.mount unit and get the default size limit of 1/2 of memory. If I want to reduce that limit, I either have to fiddle with a systemd override or else put a line in /etc/fstab just as in the days before systemd, and the systemd unit will respect the size option there.
But, that line in /etc/fstab can do the job on its own, so what's the point of enabling the tmp.mount unit?