Structuring data, data transportation...
XML is just a language like PHP. It's meant to hold data, and keep it in some sort of structure. There's no "one use" for XML. You could use it as a format to export tables (SQL or Excel) or use it as a list of items in a shopping cart, or do a back-up of private messages / emails, or even just use it as a basic layout of a page (with the help of XSLT). There are many many uses for it.
