XML is usually used to put together a whole bunch of settings in one file, and then upload the file to a certain program and you easily have all of the settings...for example, if you were doing a mod for IPB, you could upload the XML file to the components, add in a few php file edits and your done...XML makes things easier than having to go in and do things manually...
as a language itself, its really just a way to share data over information systems...
basically, its a shortcut...