Drupal comes with a great user management system. Users can be grouped into types such as "Backpackers", "Deal seekers", "Luxury likers" so that you can target specific content or advertising to specific user groups. Groups such as "editor", "designer", "SEO specialist", "admin" also allow you to restrict certain tasks and functionality to certain user groups. Imagine you want someone to write content for your site, obviously you do not want to give them access to your user list or admin settings just in case, with Drupal you can set privileges on a group per group basis.
You can even set on a page per page basis what group of users can see the pages. Drupal also recognises user privileges when building the menu meaning users do not click through pages seeing "access denied".