Someone who knows you could steal your phone, go near the front door of your house/apartment to be within wifi reach, login to your phone, unlock it and then wreck havoc on your life.
Android has had this feature for years and it still requires an initial unlock of the device to remain unlocked at the location you've picked. So when I get home I still have to manually unlock my phone once to keep it unlocked. As an extra safety measure the phone locks itself again after 4 hours.