> If you're not supposed to put something there, why does the field exist?

The field exists because it is sometimes useful to identify yourself. But because it is sometimes useful to identify yourself doesn't mean it's always useful to identify yourself. Most of the time, it's just pretentiousness.

Most web-based applications that ask for identity, do it without valid justification, which significantly reduces the need for something like OpenID.

In fact, what most web-based applications need is an authenticator.

If the user creates a resource, maintaining authorization to manipulate (delete, edit, append, etc) that resource later is obviously valuable. The application doesn't need to know who I am in order to do that, they just need to know that I created that object. They can do this with an authenticator- say by recording an encrypted capability string with the object.

If the user needs to prove that they created that resource, they can do so easily. They can also publish the authenticator (or destroy the authenticator) so that nobody can claim to have created the resource.