Friendly URLs are are usability patterns if you want. Let me explain myself for a moment here.
When a user sees in his browser example.com?index.aspx?action=art&id=7799&cat=52&p=2#a22 he simply ignores that. That URL doesn’t give him any information at all. It’s a bunch of numbers, parameters, crazy signs.
When he sees example.com/kitchens/a-article-about-nice-kitchen-furniture.html#comment-no-22 on the other hand, the user kindda’ guesses that he’s about to read an article about kitchen furniture, part of the kitchen articles category in the site. Hell, some computer-savy guys might know the browser will set on the 22nd comment to that article.
URLs can act like breadcrumbs. It’s a signpost, a “You are here” marker.
Bonus: they are great for SEO, everybody knows that, and they are much more appealing esthetically.