How can two-step verification increase security?
If a person has my device and my password how can two-step verification in any way increase my security? Wouldn't using my device ID provide the same security?
Two-Factor Authentication protects your Apple ID, not your devices. Your devices should be protected with a strong passcode and no one but you should have that passcode for any reason.
If a person has your device's passcode they can access the device but they cannot access your Apple ID or add other devices to the Apple ID.
2-Factor Authentication prevents anyone from accessing your account even if they have your email and password for it.