The XML is simply a very easy language that doesn’t replace any current language but enhance some capabilities. The XML is only used for the transport and the storage of the data. It enhances and helps a lot the HTML in the web development but as I said it doesn’t replace it. For writing the html code it is so easy as it doesn’t have any predefined opening or closing tags, it can only be defined by the programmer or the one writing it.
So how does that help?
Simply if you build a web application with a dynamic data it will be hard, tiring and waste of time to write for each thing an html page to load the new data. Or you can do it the easy and the more efficient way, is to write your data in an external XML file and let your html page deal with these files. This can be easily done using the JavaScript in your web application.
What the XML really offers is to transport your data to any kind of machine where it can get the XML file and display it in any way its software is designed to.
simply and good explain