Once any document has been marked up electronically it is essentially preserved for all posterity, it can be duplicated, distributed and manipulated. However it remains a flat document, and to get useful information from it requires reading and perusal by a human agent.
Marking up a document in HTML, or better yet XHTML means that the document can be displayed and printed out for easier reading. However XHTML is primarily a structural markup language that describes the structure of a document. Unless extensive use is made of the 'class' or 'title' attributes, the markup does not describe the cotent of the document. This is where XML comes in.
XML allows us to describe the content of the document. Once a document is marked up in a descriptive manner it can not only be displayed just like HTML, but it also becomes easier to retrieve information from the document, such as chapter titles etc. Certainly marking up a document in XHTML is a big advance, but marking it up in XML is an even bigger advance!