I have been working with XML since it was a glimmer in the eye of Jon Bosak. In fact, before XML was conceived, there was SGML; going from SGML to XML represented a streamlining for the web, but at its core there was not much functional difference; in fact XML is a subset of SGML. The key concept of semantic markup is central to the core value of SGML/XML.
The two main perspectives I have seen are Document-centric XML and Data-centric XML. SGML initially appeared in support of document-centric work: managing all the technical documents or contracts of IBM or Boeing, for example. Charles Goldfarb has maintained that “SGML literally makes the infrastructure of modern society possible” and I think he’s right – hmm, should we blame him for the lengths to which humans have gone to destroy the earth?