This is a question often asked by prospective web hosting customers who either have an existing web site or are starting up from scratch.
WordPress originally evolved for use on blogging sites, sites that allowed the owners to post news, articles and thoughts with a minimum “technical” requirement. Over time those same features have evolved to form the basis of a CMS (Content Management System). For business and personal web sites this allows the site owners and other nominated users to create and edit content on their web site via a simple web interface.
Whilst WordPress isn’t as fully functional a CMS as the more complete (and complex) Joomla and Drupal solutions for most of our customers WordPress is the most accessible. Experience has shown that whilst a Joomla based web site can offer a multitude of features and options this is off putting for most customers. The key to real life CMS for most site owners is that changes can be made simply and quickly.
Simply in terms of support requests, very few of our customers have been comfortable with making changes to content or adding new pages/posts to Joomla or Drupal based sites. Often we are asked to make changes for them which, while it is part of the service we offer our web hosting customers, it kind of defeats the purpose of a CMS. Owners of WordPress site owners, by contrast, seem far more comfortable with making their own changes to their site content. Support requests tend to be more related to small formatting problems.
From a development and support point of view, in most cases the WordPress architecture allows for quicker development of both themes and custom plugins to allow site specific functionality where it is needed. A normal PSD2WP conversion (converting your page design from an image or powerpoint slide to a WordPress theme) can be completed in less than a day.
For our customers this reduces the cost of creating and maintaining a site dramatically compared to other CMS options. For most SMEs WordPress more than meets their requirements, giving a low cost of ownership. Some large enterprises have also used WordPress as a basis for their sites although the trade off between ease of use and functionality is less marked when you have your own IT resource.