When aggregated with the output of popularity-contest from many other systems, this information is valuable because it can be used to determine which Debian packages are commonly installed, used, or installed and never used. This helps Debian maintainers make decisions such as which packages should be installed by default on new systems.
The resulting statistic is available from the project home page http://popcon.debian.org/.
Normally, popularity-contest is run from a cron(8) job, /etc/cron.daily/popularity-contest, which automatically submits the results to Debian package maintainers (only once a week) according to the settings in /etc/popularity-contest.conf and /usr/share/popularity-contest/default.conf.
Additional documentation is in /usr/share/doc/popularity-contest/.