There are incentives to have XML interfaces languages. For example, XUL, XBL are examples. So when there’s a good practice on a topic, it’s good to standardize it then there’s a clear reference for future use and more a Royalty Free reference to implement by anyone

Another aspect is AJAX like applications or XMLHttpRequest, which has no specification for now. Most of the development are a reverse engineering of the initial feature implemented in IE. Having a well defined specification for this will promote interoperability and then people will be able to develop applications in a clear environment.

Hope it helps to understand.