Well, to see some of the benefits of XML you have to have a particular language written in XML to see.
For instance, XHTML is HTML reformulated as XML. It's an XML language for marking up text with links, pictures, emphasis, etc.
SVG is a graphics language written in XML.
XSLT lets you transform one language into another.
The list goes on and on. The key question is what sort of data you want to store.