As bill said it is separation of particular concern.
To demonstrate it properly, we have used a Tuckey's UrlRewrite filter. Our requirement is show the URL's in SEO friendy manner like www.nissan.com/Acura/MDX instead of www.nissan.com?make=Acura&model=MDX.
The job of the filter is to convert the SEO friendly url to Server understandable URL. This made our pages get ranked more by Search Engines and improved the readability.