What is XML useful for in websites w/ minimal external data?

:?: I'm just starting to look at XML and XHTML. From what I understand, XHTML is basically HTML with stricter guidelines - which I think is great.
However, although I understand that XML features extensibility, and I can define my own tags, what use does this have for me? A web developer?
I can sort of see how it could aid in creating cross-platform file formats...
but I'm really unclear as to how I can use it. :?:

Please advise - thank you.