Many of the OpenID providers through which a user can log in to your site or service may have a large amount of existing data about the user and his linked account.
Using OpenID extensions, providers can permit an implementing site to pull profile information from their memberships systems, allowing the implementer to prepopulate its own user profile system. This is another important benefit of OpenID and increases the service’s ease of use.
These extensions allow a consumer site to not only leverage a user’s existing profile information, but the profile may also link to additional user accounts and interactions with other sites, services, or users. Having access to these sources will help you deliver a comprehensive social graph for the user right when he starts using your service.