12. One big upgrade of drupal core can be done as opposed to updating the core code for separate installations each of which could be a different version of Drupal.
13. Modules/Themes can be shared across sites or they can have their own modules/themes.
14.
15. Updating core and contrib modules, means running update.php for each site in your Multi-Site setup. (though you might be able to use Aegir/Drush to run update.php for all sites in one go)
17. For example, for a fictitious site installed at http://www.drupal.org/mysite/test/, the 'settings.php' is searched in the following directories: 1. sites/www.drupal.org.mysite.test 2. sites/drupal.org.mysite.test 3. sites/org.mysite.test 4. sites/www.drupal.org.mysite 5. sites/drupal.org.mysite 6. sites/org.mysite 7. sites/www.drupal.org 8. sites/drupal.org 9. sites/org 10. sites/default