This site has the spider friendly urls on.
So compare your site main page to this one to see the difference.
It just makes it easier for the spiders to crawl the site since some don't like the standard url like ?,= BLA BLA and they give up.
I have never seen any cpu hit from doing so.
But I would not suggest turning it on and off on a I feel like trying it today.
Because it will confuse people especially when they google something and they see two results with different url's to the same site.
It has been quite a while since I tried but if you enable the option the old and new url will both return the same post.
