This is particularly efficient for large corporations engaging multi-sites. Here users with access permissions can move from the main public facing site to intranet to administrative, etc... with one-click. When the user signs-out a single action terminates access to all the systems. Benefits of using single sign-on include:
Reducing password fatigue from different user name and password combinations;
Reducing time spent re-entering passwords for the same identity;
Reducing IT costs due to lower number of IT help desk calls about passwords.
SSO can also share authentication with other applications and systems (such as facebook, twitter, etc.) and combines this with techniques to ensure that users do not have to actively enter their credentials more than once.
Web administrators and managers, who maintain several websites, simplify administration by sharing one instance of the framework on a single server. Apache mod re-write allows you to use the same codebase to respond to different requests. As each site has a different database, sites sharing the same codebase share the same files, modules and themes. The site you are viewing now is a multi-site with single sign on.