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I found a good explanation at http://www.alistapart.com/

"The reason that we use XML instead of a specific application is that
XML is not just a pretty face, living in isolation from the rest of the
computing world. XML is more than a rulebook for generating custom
markup languages. It is part of a family of technologies, which, working
together, make your XML-based documents very useful indeed."

http://www.alistapart.com/stories/usingxml/