We use WordPress due to the ease of use for clients who want to take control of their websites and be able to update and publish new content by themselves. WordPress is as easy to use as editing a Microsoft Word document.
WordPress started out as a blogging application and has turned into the most powerful publishing platform on the Web today. WordPress is one of the most vibrant of the Open Source communities and has many powerful add-ons, many of them available for free.
Using WordPress increases getting found by Google in your relevant keyword space.