"Consider this scenario... You are interested in installing GNU/Linux on your machine. Assuming you already have the latest version burned on to a CD, it is a simple affair of popping the CD into your CD drive and starting the installation. But once the installation is done and finished, you will most certainly want to install additional software apart from the ones bundled with the CD. And if you are using a Debian based Linux distribution such as Ubuntu, you will be using the apt-get method. Over a period of time you would have installed a number of additional software including any packages satisfying their dependencies as well as upgraded some of the software to the most recent version..."