James Tauber explains:
<XML is fundamentally (...) about structured documents.
A markup language is just a convention for adding information to and about
the content of a document.
Tags provide a means of expressing the hierarchial structure of documents
and associating attribute-value pairs with nodes on that hierarchy.
XML provides a serialisation of such structure that takes into
consideration both machine and human readability. Unlike languages like
PDF, XML can be produced easily by hand.>
Thanks to Chris Senior for asking and to Jas. Tauber for spelling this out
so clearly. The advantage of machine and human readability seems an
essential one that I've just begun to understand.