You need a phone to make and receive phone calls, among other things. That's why you already have it. You don't need the key for anything but opening your front door. The point of this device is to obviate the need to carry that key all day for that single moment it's needed.

Nobody's going to find your emergency spare key if it's left in a non-obvious place; there are no rings of thieves with metal detectors scouring half acre properties.