Drupal primary menus used for top navigation dropdowns and sidebar local navigation
On a site I am working on, the theme is Acquia Prosper. The Primary Menu for the top navigation uses Superfish dropdowns if you turn them on. If you want a menu item to have a dropdown, you need to make sure that menu item is set to Expanded.
However, in many situations, you will also use the Primary menu for local navigation, so users can easily see what section of the site they are in, what page they are on and what other menu items are in that section.