let me first list down the advantages with which i'll tell the applications,
1.platform independent,with a commom set DTD different set of systems and companies communicate.
**** It does'nt matter what type of database in use,everything will be in xml files,so all our operations will be to that xml files.so the heirarchy will be database->business rules->xml->client pages. so that every function will not go to db,which improves performance.it may hold for any kind of db software.
2.searching,it is definitely going to provide a efficient way out.
*** now since all are user defined tags,there will be some meaning,so searches can be specific and fast
3.then customized web pages,bcoz content and format or separated,so we can give customized ,preferred data to user.
xml and xsl.
4.and much more CDF,OSD,OFX,OTP,EDI
5.some experts predict if xml used properly,it will be the database for the web.
i hope this will be tiny contribution from me,for you to start