Most of the WordPress users have more than one site. Even if it is only a personal blog, or a presentation website. Usually, webmasters are having 5-10 websites to manage at one time. Few of them are websites for main businesses, others are blogs, presentation websites or feeder blogs for main websites. In all cases, it is hard to update all of them whenever an update appears, or whenever you want to add a nice widget that you found on the web.
The standard solution for WordPress is the multi-user version. However, it is not a very good idea to have many websites under the same installation and on the same hosting, so the wordpress multi user is intended to be used by more users, not for more websites. The right solution for this situations is ManageWP. A web application that will let you manage all your wordpress sites from the same place.
With ManageWP, you can manage all your wordpress site from one place, you can install, upgrade and monitor more websites at the same time, backup all your wordpress sites at once and publish posts to all of your blogs at once.