Why XML versions of Web sites?

[LRW]

I've noticed some sites posting links to XML versions of their Web
sites, like:
gamegirladvance.com has http://www.gamegirladvance.com/index.rdf
lockergnome.com has http://www.lockergnome.com/lockergnome.xml and the
like.

Why? What is it used for? To what purpose? I'm a neophyte Wed
designer, but know nothing about XML (aside from what Sam's "Teach
Yourself XML" might have in it's intro chapter,) so I'm really
curious. I'd like to know if it's something I should become savvy
about and start incorporating myself.

Thanks for any clue-by-fours!
Liam