Modern websites are highly complex online applications. Information is stored in a database where it can be manipulated, quizzed and displayed on the front end of the website. The CMS is the engine which makes all of this happen.
There was a time when a web page was just an html file - a bit of text and maybe a few images. The modern web is very different. When a user loads a page on your website we will serve up content specific to that user packaged in a layout designed to fit the requirements of whatever device they may be using. Their movements will be tracked and analyzed and their actions might trigger alerts or emails. A web page might look different for every user.
For the past 6 or 7 years we have been using Drupal and watching it grow as a platform. It's an immensely powerful system capable of handling sites as complex as weather.com and whitehouse.gov.
Drupal CMS enables you to control your data with publishing workflows. Set up roles for content authors and editors and make sure nothing makes the front of the site before it has been properly authorised. Revert content to any of the previously saved iterations. Schedule back ups to multiple locations.
Because it's open source we can take it apart and rebuild it if we need to and it has the support of an enormous global developer community contributing their ideas back into the core.