The Magento theme uses two very powerful elements, with which you can modify most of the HTML-output of the Magento system: XML-layouts and PHTML-templates. Core hacks are very bad, so Magento allows you to override each file within your own Magento theme.
While this is no problem with PHTML-templates, XML-layouts contain many times complicated structures. If you could add your own custom XML-layout file, you allow have to manage this file - separating your own changes from the core-files. This tutorial shows you how to build a custom Magento module, that only adds its own XML-layout file - so you can safely add XML-updates to it.
Read more: Build your own Magento override-module