The reason for that is to support the use case (which is actually my scenario) when the CAS server lives on the same domain/port as the web application (for example, both the CAS server and the app are behind a load balancer). So if you go to http://foo.com:8080/webapp the CAS server url would be http://foo.com:8080/cas, and if you go to http://bar.com:8080/webapp the CAS server url would be http://bar.com:8080/cas. Does that make sense?