When it started out, systemd was in many ways relentlessly pragmatic. My shining example of this is that the developers went to fairly great lengths to integrate both System V init scripts and
/etc/fstab into systemd in a fairly deep and thus quite convenient way. The easy way would have been to just run things and mount filesystems through some compatibility shims and programs. Systemd went the extra distance to make them more or less real units, which means that you can do things like add extra dependencies to System V init scripts through
/etc/systemd/system overrides, just as if they were native systemd units.
(This has not always worked seamlessly, especially for mounts, but it has gotten better over time.)
As well as being convenient for people using systemd, I suspect that this was a pragmatic decision. Being a better System V init than SysV init itself undoubtedly didn’t hurt systemd’s case to be the winning init system; it gave people a few more reasons to like systemd and approve of it and maybe even push for it.