More than likely, an application requires more information about the user during authentication (i.e. a list of groups to use for authorization decisions or the user's preferred language to personalize the application) these additional attributes can be provided by the authentication provider during the authentication event or through an out-of-band process (i.e. federated provisioning (SCIM) or via an API call like the OpenID Connect UserInfo API).
An application may have an application data store that contains application-specific information about the incoming user. For some applications the only information the application needs from the authentication provider is the username. It can then map that user to their application profile to associate the user with their application profile and authorizations.