The /etc/inittab has some special features that allow init to react to special circumstances. These special features are marked by special keywords in the third field. Some examples:
Allows init to shut the system down, when the power fails. This assumes the use of a UPS, and software that watches the UPS and informs init that the power is off.
Allows init to reboot the system, when the user presses ctrl-alt-del on the console keyboard. Note that the system administrator can configure the reaction to ctrl-alt-del to be something else instead, e.g., to be ignored, if the system is in a public location. (Or to start nethack.)
Command to be run when the system is booted. This command usually cleans up /tmp, for example.