Internal Drupal Statistics
There are various analytic tools available to gain insight in your sites' statistics. These are mostly tools like Clicky, Google Analytics or Crazy Egg and only give you information about your visitors.
There are often times when you need to know what is happening within your content management system itself. Especially if you are running a dynamic community website with lots of user activity. How is my content devided and which content-types are popular? How many users do I have which never log in? And a lot of other questions.
Most of the time, these are internal Drupal statistics, and while you can get some of these things to work with third party services (like role segregation in Google Analytics), not everything can be accessed by these tools.
For Drupal, there is an alternative called the Report module. Still an alpha at this point, but it delivers some great info in addition to the core statistics module. The beauty of this module is the way it works together with the Chart API module to generate nice charts and diagrams. Like in the example above.