Why not just init? What do /etc/rc*.* scripts, rc*.*d and init*.*d directories and directory trees, etc (ex like svc, startsrc, and on and on) do that init and inittab do not or could not do? Is it just so novice SysAdmins need not mess with init?
Is there an overview anyplace of how start-up evolved and developed in Unix? Is there an authoritative description/comparison of the startup/shutdown/process-management regimens as they exist today in the various Unixes? (Unices?)
Some of it is structural, as there is a different level of service from the O/S when different scripts are called, and each script raises that level of service. When new daemons need to be turned on, they each have an appropriate time to start, before they are needed and after they can get what they need. But some of it is probably accidental, grew like topsy!
init is probably the closest you can get to a "standard" daemon manager.
Solaris for instance has Service Management Facility (SMF) and milestones instead of runlevels, AIX has System Resource Controller (SRC) and it's own set of tools for it, some "cutting-edge" Linux flavours like Fedora don't even use init and have completely replaced it in favor of "systemd", MacOS has "launchd" instead of init, and the list continues... you get the picture.
Why do debian and red hat have different package managers? Why are there different desktop window managers? Different projects got started and they took different approaches.
I doubt there is a source that describes all the different startup regimens in detail. It would seem a pretty tedious task to compile and compare them.
UNIX admin is never addressed by POSIX. To get an idea look at the UNIX Rosetta Stone: