WordPress is a CMS. The whole idea being to manage content and make websites editable without having to wrangle code. Any theme can handle Posts and Pages, but what about those "smaller" areas. Little chunks of text placed around a design like a small "about" section, or the copy in the footer of a website. A lot of times this text is hard-baked right into the theme, which isn't a very friendly way to do things, as it can't be updated by average Joe user.