Let's say you want to use a single Drupal install to run your company's primary web site and your company's blog site (multisite), but you also have different themes and navigation hierarchies for the two sites. Sure, there are some Drupal modules that may provide most of this functionality. However, you can accomplish exactly the same thing with a plain vanilla Drupal install and the Pathauto module (a module that you should really be using anyways).