So you want to parse a website?
Python has some pretty cool html, rss, and atom parsing modules. There is also a promising CSS parser. This parser is cssutils.
Parsing CSS is one part which can occasionally come in handy. Say you want to figure out how big a piece of text will display on a page? Without CSS parsing you can not test this.
It's also handy if you are using CSS in your own non web based GUI engine. CSS is a fairly well understood language. So using it instead of your own format might be a good idea.
Above are some reasons why you might want to parse CSS with python. However I'm sure there are others which might pop into your head if you are a web developer.
Thanks to Christof Hoeke for providing one of the missing batteries for a modern WWW (cssutils).