This would be the default context to use for any unauthenticated calls. In other words, that context parameter is telling the Internal SIP profile which dialplan context to use when an unauth'd call comes in. Typically this would be inbound SIP provider calls, such as calls in to a DID number. You definitely want unauth'd calls to be handled by your public/untrusted/paranoid dialplan context. To paraphrase Brian West: "paranoid by default" is a good policy. If the user is authenticated then the user_context value is used.
-MC