Week 9 Xpath

Today was a lesson about Xpath, It seem it is getting more complicated. Xpath is created to search XML document. Why would I want to use XML to create searchable information? I need to read more about this topic because I have spent last 2 year learning about database. Now I have to learn a new way of searching document using Xpath, am sure it has its advantage but I remain to be convinced. I know now there are different kind of mark-up language hence the name extensible, what am wondering now is how many other extensible language are in existence or is extensibility of XML is never ending.