When the seeds of Drupal began in the late 1990s, no one (not even its creator) envisioned it as a CMS. In fact, it began as message board software in Europe.
Since becoming an open-source project, however, it gradually has become a platform for (ahem) sophisticated tastes. That is to say, WordPress and Joomla both have large communities, but Drupal has supporters that develop on the highest level.
Not only is Drupal powerful, it's also slightly more complex to implement and use. The trade-off allows for powerful functionality, however, and it lends itself exceptionally well to complex websites that require more than the usual out-of-the-box possibilities.
Another reason clients appreciate Drupal development is its excellent security rating. Since web data can be sensitive (and no one likes backend problems), a great security rating is actually a really good reason to choose a platform.
And while some have been intimidated by Drupal's tech-head reputation, it actually responds great to customization, meaning that whatever is confusing or unnecessary can be hidden or modified into something more meaningful.
All in all, it's a fantastic platform where possibilities are virtually limitless without having to break the bank along the way.