When you get into a situation that you want to change your company's email id or some other content of your website, you have to phone the techinical company to make the change. Maybe he is out-of-station or he might charge you for the simple changes made. And when you have many pages in your site, say 50 pages, you will find it very difficult to manage each page yourself.
The solution to all this is - Use a Content Management System (CMS) to manage the site content. For a static html site, information is stored in files, and for CMS, it is stored in a database. Storing in a database has many advantages. We can view a selected item, sort the items in an order, or search the entire items for a particular record. Apart from that, a CMS has an interface to edit/create contents in your site. With a good CMS integration, even 1,00,000 pages in your site will be very easy to manage.
There are many CMS available, Drupal, Joomla, Wordpres, etc. are some of the Opensource Php Content Management Systems. We use Drupal CMS to develop websites. The main reason is that Drupal is very stable. From our experience, we haven't had much management issues with this CMS. Once set up, it works fine, with no database crashes and no performance problems. One of our clients has not reported a problem even after 4 years of its implementation. And for another client, we have witnessed a spike in the traffic, with around 4000 visitors an hour for two continuous day, and yet, the load on the server was very little, which proves Drupal's excellent caching mechanism. www.drupal.org itself has close to 9,00,000 pages (http://drupal.org/node/857776, this is the last page at the time of this writing. You can have a look at page number 857776)
There are many high quality websites built in Drupal. Some of the popular sites are :
They all use Drupal to maintain the quality. We ourselves use it for building quality websites for our clients.