System can have multiple roots each with its own independent navigation. For example all pages under the home page can be independent from pages under help. Adding pages under help will affect only the navigation of the help area. This allows greater flexibility.
Each root can have a different default design.
A separate root can have your log-in form for administrators if you choose not to have open user registration.
A separate root could have your intranet or an area that includes only content for your business partners.
