Because everything, including space, is more fun when you can automate it
Intended to be the full contents of a Script/ folder, located at Kerbal Space Program/Ships/Script. (Note that the Kerbal Space Program folder is the one that contains things like GameData/, KSP_Data/, saves/.)
Once added, and KOS is installed, a KOS CPU will let you choose boot.ks while in the VAB. Then when you take your rocket to the launchpad, it will open the KOS terminal and prompt you to choose from its functions.