That's up to you. How do you need to use it? If you can't think of a scenario, perhaps you don't need it. My own experience with XML is reasonably limited, but one way in which I've used it is to create feeds, pass data in a standard format to other sites. It can be used as request output, which the user then parses with tools such as simpleXML.
With regard to schemas, you could be better off with SQL dump files. I find json a more useful format these days, but again, it really does depend on the data and what needs to be done with it.