Why do I write my web site in XML, and not in HTML or (your favorite authoring tool here)?
To be honest, the main reason is because I always have fun with technlogy. I'm a mathematician and I like things because of their structure. XML is a technology which allows to focus on content and structure. The web is full of noisy stuff without much content. I want content without much noise. Writing the content in XML is one way to accomplish this. This allows me to have a complete separation of content and presentation. If necessary, I can change the complete layout of the web site without changing a single bit of the content.
 Of course, there are more ways to go to Rome.