Here's a practical example:
I have some clients who don't want to or don't have time to use a DB admin to update theirselves their data on their web page. Shops owners, small and medium trade companies...They usualy have their data (items, prices, and so on) stored on Excell files. It is easier for them to save their XLS in XML format and send it FTP to the server than opening an Admin, even a friendly HTML build one, in order to do changes here and there.
Now everything I have to do was to find a dynamic way (in fact I use AJAX) to grab data from that XML file and convert it in a HTML format to be displayed on a web page. So that now neither me nor my client have to update "manually" the web pages. He simply save it's data in XML format and sends it to the server.
As NogDog says - "exchange information between different applications that might reside on disparate platforms"