So, of course, at this precise moment your phone is unlocked simply because you are using it.
You have taken the precaution set up a SIM card PIN however, so that even if someone steals your locked phone and puts your SIM card into their own unlocked phone, they will not be able to steal from you by using your phone number to receive SMS password reset messages or call premium rate numbers. You think you are pretty safe...
But now, all of a sudden, someone snatches your phone out of your hand and since they now have your unlocked phone, they can easily access your SMS messages and emails. This means they can now reset your bank PINs and email passwords which then locks you out of your own accounts and gives them full access instead of you! Google will helpfully fill in any of your passwords! Two factor authentication is not a problem because the second factor is your phone!