This recipe is not limited to running only applications or services at startup, but to also start scripts on bootup (power on) of a system. For example, if your system boots up and you would like to apply several tweaks to the OS such as performance enhancements or battery tweaks, you can do this on startup via the systemd or init.d script. Another example could be to run a never ending script that creates logging events, like an electronic version of a pulse monitor.
In short, Linux or most *NIX systems use either the venerable rc.d system or the newer and more controversial systemd system to manage the starting and stopping of system resources. Without diving into the entire boot sequence of Linux, here ...
