The most common scenarios where developers want greater
flexibility with URLs are:
1) Handling cases where you want to restructure the pages
within your web application, and you want to ensure that people who have
bookmarked old URLs don't break when you move pages around. Url-rewriting
enables you to transparently forward requests to the new page location without
breaking browsers.
2) Improving the search relevancy of pages on your site with
search engines like Google, Yahoo and Live. Specifically, URL Rewriting
can often make it easier to embed common keywords into the URLs of the pages on
your sites, which can often increase the chance of someone clicking your
link. Moving from using querystring arguments to instead use fully
qualified URL's can also in some cases increase your priority in
search engine results. Using techniques that force referring links to use
the same case and URL entrypoint (for example: weblogs.asp.net/scottgu instead
of weblogs.asp.net/scottgu/default.aspx) can also avoid diluting your pagerank
across multiple URLs, and increase your search results.
In a world where search engines increasingly drive traffic
to sites, extracting any little improvement in your page ranking can yield very
good ROI to your business. Increasingly this is driving developers to use
URL-Rewriting and other SEO (search engine optimization) techniques to optimize
sites (note that SEO is a fast moving space, and the recommendations for
increasing your search relevancy evolve monthly). For a list of some good
search engine optimization suggestions, I'd recommend reading the SSW Rules to Better Google Rankings, as well as
MarketPosition's article on how URLs can affect top search engine ranking.