Preparing the APT packaging system for your environment (Simple)

APT is a mature part of Debian that became the distribution's most important differentiator for years, enabling hundreds of derivative projects, including Ubuntu. Since free software is distributed by multiple parties, not always in full coordination in terms of packaging formats, release dates, versioning, configuration files, and so on, distributions are expected to put together the broadest set of software that makes sense for most users in a consistent way.

APT provides an easy, robust way for users to install software and have Debian take care of pulling software from Internet mirrors, sorting out dependencies and conflicts, installing the software and making basic configuration choices. That's why it's important.