because whilst systemd-initd is the part that everyone is generally aware of, that’s linked to systemd-logind so that processes can be started as different users… process init, session management, and user management are intertwined
they don’t have to be for sure - sysv init proves that - but in modern linux, they are and that comes with a load of benefits
https://deepwiki.com/systemd/systemd/6-user-and-session-management