HELP ! why / when use XML?

Okay people,
I’ve searched this forum, but non of the posts could clarify this for me. I generally now what XML is about( a sort of content oriented, by custom tags well structured textfile) . But what is the big fuss about? I don’t see why it’s better than using a PHP or ASP script to get data out of a MySql database, or for smaller chunks of data just use an URL encoded txt-file.

Simply put: WHY use xml? And WHEN do you use it?

In what situation do I choose XML because it’s much handier/better/… than the other technologies?

I’ve missed out on something I guess.
Please help

thanks guys
Stanley