Why use Debian? There are plenty of philosophical reasons: the legendary Debian social contract, the community, and all included software in the repos happens to be free (as in freedom), long a hallmark of Debian.
The more practical appeal of Debian lies in its legendary stability. I’ve been running Debian servers since 2005 (Sarge) and have never had a server crash. This dependability is part of the reason Debian is the base for dozens of downstream distros.