From Joannes Vermorel's blog:
Do not break URLs: When migrating your web pages, it's important to include the proper redirections. I have been using UrlRewritingNet, a nice open-source URL rewriting tool. UrlRewritingNet is very convenient to migrate ASP.NET pages because ISS does not intercept the requests even before they hit the ASP.NET machine.
From Scott Guthrie’s blog:
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.