I just read the sample chapters of the new XML book, and im kinda confused, what is the advantage of XML over plain XHTML?

The book states that it is easier to display documents in mobile devices using XML, and that is easier to translate content language. But, it doesn't demonstrate either of them, nor gives any other reasons to use XML. I find the XML concept intresting and it could be the next big thing...but currently i cant see why i should use it over XHTML and PHP...

I'm kinda sure that its very easy to implement a template system and different languagefiles in PHP and XHTML only. And if XHTML is designed to just make it easy to make web documents, I cant see why i should use XML if i dont plan to view my site in mobile devices...

Anyone care to enlighten me ?