Although never fully intended as a CMS, WordPress is indeed one of the most widely used CMS's in the world. If you've built websites for clients using WordPress, you've probably encountered some difficulties in training your clients on how to use WordPress properly in order to update the content of their website. You're no doubt wasting valuable time answering the same questions over and over again.
There are more than 75 million websites running WordPress. More than 20,000 people make their living full time with WordPress in some way (designers, developers, consultants). In between the 20,000 and 75 million are a great number of people that maintain their own website with varying degrees of experience (and curiosity).