Okay let me explain my understanding of why to use XML well one use any way.
XML is used to define the structure of a document, what the document should contain but not how it is formatted. So XML is sort of a blue print for how other documents such as HTML, text and PDF should be structured and what information they should contain.
An examle of two ecommerce systems: eshop[a] has a list of products with their prices in an XML file. eshop has set up a deal with eshop[a] to sell eshop[a]'s products on eshop's website. So using a parser eshop converts the XML file using PHP (or other language) into HTML to display the products and prices that eshop[a] has to offer. Then when eshop[a] updates the XML file the information is automatically updated on eshop's website.
Is this how it all works, generally... basically...? Am I right? :o