Combining user accounts with client registration pages for an API implementation web site integrates the developer and the user. Development requires an account to test the user page and registration combines the step for the two. Besides both are the company's assets. A client registration page has details such as an account for registering, a display name which is the name that others will see, an e-mail where the validations are sent to. Validations are required to confirm that the data entry is not automated. Other details include the password for the account and password security requirements enforcement, the details for the name of the application, the web site, what the application will do. The application description is important because it informs what the application intends to do. This we use to validate the client applications when it is being approved.
Also a callback URL that is registered. This callback URL is important for all OAuth client validations and hence this is critical to the OAuth logins from the client. The OAuth spec demands that the client redirect URLs be validated so this is important.