I think the best answer is that systemd-the-project tries to be the things that all of the different distros historically had to do themselves, the things that didn't have an upstream, but maybe deserved to have an upstream so that they could be reused by other distros, and maybe benefit from more eyes and more engineering effort.

Each distro used to have to roll their own initscript system (with varying degrees of compatibility), so they made `systemd`. Each distro had their own init script to save/load the backlight state, so they made `systemd-backlight` (if you're going to replace the initscript system, you'll also have to replace the things that the initscripts for that system did). Each distro used to have to roll their own network configuration scripts (Debian's /etc/network, Arch Linux's netcfg and later netctl, ...), so they made `systemd-networkd`.

Some of this class of thing got picked up by other projects; each distro used to have to roll its own initramfs system; today Dracut exists, so systemd won't be doing that (unless the Dracut folks decide to team up with the systemd folks--remember gummiboot? gummiboot renamed to systemd-boot).

In one of your other comments you wrote "in my opinion that implies more modularity than it posesses." While there are some nasty couplings (it'll take some patching to get systemd-nspawn to work on a non-systemd system), you can successfully use many of the systemd modules on an OpenRC system; it is a lot more modular than people give it credit for.