I'm a Debian user. One of the strongest points in the Debian world is the package management system that is both binary and source based. In addition to the software, Debian maintains strict rules regarding the package management process to ensure high-quality packages with the minimum of conflicts.

Sometimes, however, those conflicts occur especially running the "rolling" testing or unstable distros. Frequently the issues come when updating "sets" that contain many packages, such as KDE or Gnome, when one piece didn't build but other chunks are there. Being able to pull down the source code and build it on your own system before it hits your mirror can be a lifesaver.

In addition to building missing packages, Debian's source based repositories are the only "safe" and "easy" way to mix sources - something I've personally had to do to leverage the large number of packages in GetDeb that are designed for Ubuntu but will build and play nice with Debian systems.

I've been meaning for a LONG time to sit and write out a tutorial on how to easily use the tools, but someone else has done basically that so I'll merely link there and call it a day. :)

http://www.ducea.com/2008/03/06/howto-recompile-debian-packages/