I get the impression that the primary reason for systemd using cgroups was "security". This in that it provided the means of namespace isolating daemons.

This in turn because if you follow Poetterings project history it starts with him creating Pulseaudio to handle moving between sound devices while in use, then crated Avahi to provide a means of Pulseaudio equipped computers to find each other on a network, and then came Systemd based on what he learned about daemon security while writing Avahi.

Sadly i can't shake the feeling that the security in systemd is all too often leaning towards "security theater".