Cheap, simple, effective.
Cars these days are reliant upon the electrics to operate both doors & windows.
In the event of an accident where the engine wont start & you cannot open the doors, as happened to a friend of mine recently, you can become trapped in the vehicle.
Windscreen & side window glass is designed to be tough. It can be very hard to break when its your only means of exit.
This hammer gives you that ability, combined with a small seatbelt cutter that worked very well when tested.