XML What is it good for?

Yes. Quite.

Compared to what? XML in general, and SGML in particular is good for storing semantic information within documents, for example the <strong></strong> tags which add emphasis to passages in HTML documents.

Mayor benefits of XML:
* Open Web Standard - no price of admission, can be parsed in practically every language on practically every platform.
* Widely Adopted - hundreds of tools allow you very easy access to data, in very visual tools - you do not have to be a programmer.
* Easy Integration - Using XSLT you can easily convert between different formats (for instance XML and XHTML) or between two differently structured XML documents.
* Strict Syntax - XML does not play well with people who write rubbish data. Thus using a simple validator against a given XML Shema, you can easily determine whether the entered data is incorrect or not.
* Scalable - There is no (theoretical) upper limit to how large XML documents can be, and since they're stored in a machine-readable form of plaintext, they can be read for a very, very long time.