Purpose of inittab file?

Contains the instructions for init.......

The /etc/init program starts during the last phase of kernel initialization and has a ``process id'' (PID) of ``1''. The init process starts all other processes.

The /etc/inittab file contains instructions for init.
The init program reads the inittab file under three circumstances:
at boot time,
when an init-started process completes, and
when the system administrator executes either the /etc/init command with a run-level argument. The arguments passed to init allow you to change the system run level or force init to examine the inittab file without changing the run level. When the system changes run levels, init scans inittab for instructions that apply to the new state.