Because it's nice to have a URL that is descriptive so I can get an idea of what I'm clicking on before I click on it. Maybe I've just been goatsed too many times...
I believe clean, understandable URLs also help search engines index content more appropriately (and supposedly helps rankings), but with the speed and opaqueness of exactly how indexing works I could be wrong these days.
Edit: I should have said "...of exactly how search algorithms work..."