In ASP.NET MVC, getting your routes setup properly can be tricky if you have a lot of routes. Whenever something is tricky, and might be touched often, having a set of unit tests around it can very helpful to ensure you aren't breaking it when you make changes. Lots of routes can definitely qualify; your users rely on unbroken links and stable URLs for a good experience.
The techniques shown here not only work for unit testing, but can also be used outside of the ASP.NET pipeline when you need to interact with the routing system (for example, to generate URLs from a service). The routing system used by ASP.NET MVC uses the abstraction classes, so the actual running ASP.NET pipline isn't required.