In the old version, when you log in to Apple, you get a window listing all your iOS devices that use your Apple ID. You select the one you have with you and the code is pushed to that device in a notification window. On the new version, when you log in, ALL iOS devices gets the notification and you can allow or deny. On the device you allow access on, it displays the code.
If I lose my iPhone and someone has or gets my Apple ID, then they have control of my account. If I go to log in they could potentially keep denying me access.