It’s important that public-facing websites respond to requests for both domain.tld and www.domain.tld. You can’t control what your users will type into their browsers and you never know which form of your site’s URL people will use in links that they share in email, social media, and links on their own sites. Of course, you want to be sure that your website responds even if they don’t use your preferred version of your URL.
However, it’s nearly as important that all of those requests are redirected to just one address for SEO purposes. This is known as choosing and enforcing a canonical URL. If you don’t enforce a canonical URL and a search engine indexes duplicate copies of your content, you risk diluting the authority that backlinks have given your content and you even risk incurring the dreaded duplicate content penalty. Both will impact how your content fares in search result rankings.
rel="canonical" and improvements to search engine algorithms have helped reduce unwarranted penalties related to this mistake, the risk of unnecessarily falling behind in the rankings is too great to ignore.
To solve that problem, many websites running on IIS make use of its built-in rewrite module to enforce a canonical domain name. Unfortunately, the most obvious way to accomplish that ends up causing trouble when you want to work with the site locally.