Information is hard to reuse if it is not consistent. If the information doesn't follow certain rules when it is ready for processing and display, humans will need to spend extra time checking the accuracy and quality of the delivered result.
As stated in Arbortext's article "Validation: It's a Good Thing": "If every writer contributing to a 3,000 page document gets to choose his or her own style of emphasizing foreign phrases, then every bon mot will exhibit far too much savoir faire for the poor copy editor's taste, and customers will be confused rather than enchanted."
A Document Type Definition (DTD), which requires an author to conform to predetermined structures, can prevent many human variations, and this prevent time spent editing the document after it is written. Creativity still exists, it has just "moves up a little in abstraction."