It all depends on the implementation on the relying site.
Indeed you must still pick a username (or perhaps you don't -- again, depends on the implementation), but it all comes down to simplifying the identification process. If the identity provider recognizes you, the relying site trusts that the information you provided there still holds true.
Single sign-on is the main hook right now and it's an attractive one, especially for users who are hesitant to try new services due to a lengthy sign-up form. Because it's open-spec, there are some interesting new applications for OpenID that are coming into light.
There was a good point that Simon Willison raised recently. If your building a web app and that app gets dugg, the digg users will be able to log straight into your site with no sign up process if you support openID. Aything that lowers the barriers to someone using your app has got to be a good thing.
Indeed you must still pick a username (or perhaps you don't -- again, depends on the implementation), but it all comes down to simplifying the identification process. If the identity provider recognizes you, the relying site trusts that the information you provided there still holds true.
Single sign-on is the main hook right now and it's an attractive one, especially for users who are hesitant to try new services due to a lengthy sign-up form. Because it's open-spec, there are some interesting new applications for OpenID that are coming into light.