This is useful if you live close or drive by often lakes or swamps. In case you fall in the waters and the electric windows stop working and you cannot push the car doors open. As shown in the picture, it has small pointy metal part which when pushed against a car window will shatter the window.
This has proven to be very useful. You might be able to find someone to jump start your car, but what if he/she does not have the cable? Make it easier for people when you need help!
Unless you have the road-side emergency insurance (either from your insurance company or AAA, in which case, you can call a locksmith), a spare key can save you money. Even if you can call a locksmith, it’s going to cost you a lot of time and trouble.