Drupal has an option that allows you to have one theme for administrators while your visitors are viewing another theme. If you have multiple Drupal installations or websites, this feature would allow you to have one administration theme for all of them. Another reason is that some third party Drupal themes do not render the Administration pages correctly.

By changing the them for administrators to Garland or another default theme, you take the rendering mistakes away. To use this feature, just go to Site configuration>Administration theme. From here you can choose what them you want to use for the Administrator. All other users will still see the theme you set up under Site Building>Themes.

One reason I don't like this feature is that you are looking at your Drupal site through a different set of eyes than your user, so if you use this feature, make sure you go through the site as a general user to make sure everything is rendering properly.