Journal module adds additional fields to all forms in a Drupal site to allow developers and site administrators to record and track all changes that have been performed to setup a site or alter its configuration. Journal also allows developers to maintain a log of applied patches and customizations on a Drupal site, including links to issue trackers like Drupal.org issue queues (starting from 6.x-1.3 / 5.x-1.5).
Journal is primarily useful for developers and site administrators working in a team environment. Since Drupal is a fully-fledged content management framework, it is often not easy to communicate, track and audit all changes that have been applied to a site. Even without contributed modules one is able to build a totally customized site.
Maintaining a log of patches and customizations helps in determining whether a module can be safely updated, and moreover in building sustainable, well-documented Drupal sites.
Journal's log is comparable to the watchdog log in Drupal core. However, watchdog logs can be truncated or emptied at any time, so one would loose all entries.