This sample is of an OpenID Relying Party that acts within a controlled set of web sites (perhaps all belonging to the same organization).  This particular RP is configured to require authentication for all web pages, and to always use just one (trusted) OP (the OpenIdWebRingSsoProvider) without ever prompting the user.

Although the sample OP uses Windows Authentication, and so this RP could easily do the same, the idea is that the OP and RP may exist on different network topologies, or the OP may be the only site with access to the user credential database, or any number of other scenarios where the RP doesn't have the freedom to authenticate the user the way the OP has, yet this set of web sites want to have the users only authenticate themselves to one site with one set of credentials.