The originating purpose of the Drupal Migrate module is to import content from elsewhere. Where? Wordpress .. etc. Any place outside of Drupalsphere. The good thing is that the module is fairly open about what it will import, and therefore it's possible to import into a Drupal site from a Drupal site. And that by doing so, one can upgrade a Drupal site without going through the official Upgrade process.
Why might you want to do this?