I dont have time for a long reply but basically consider this forum (VBulletin)for example. There are xhtml/css coders that do not understand php and mysql. To over come those troubles xml is used as the template engine for it. Other uses is banks may use it, say an xml file is sent from one program and there program will be able to understand that information no matter what language each application was done in.

Thats all I have time for... Some one else can expand