At least on an Android device, and I assume on an iPhone, there are certain actions you can take without entering a password, such as answering a call.
So if someone called me and I picked up and started talking, and Scotland Yard swiped my phone, they wouldn’t get anywhere because they hadn’t entered the password yet. Exiting the call screen would return them to the lock screen.
Phones should definitely extend this to allow you to customize the location of the boundary. It would be a simple change to allow you to make phone calls as well without having to enter the password. Or have different passwords for different functions.
