For years I've had a hard time explaining to people that there was a difference between the text that an author creates and the rendering of the text on the page. In the outlining world it's obvious, I create text with a structure that is reflected in HTML in any number of different ways, yet the original text, the source code, only represents structure in one way. Since most people don't write in outliners, the idea was hard to grasp.
But there was a much more obvious way to explain it, using Markdown as the example. If you've ever entered the special characters that Markdown defines, you of course get that there's a difference between the original and the rendering. So, a CMS that understands Markdown must preserve the source code for the author. If WordPress supported Markdown, for example, it must have a way of storing the source in addition to the rendered text.