At least in my jurisdiction (Norway), you install a car alarm because your insurance company still gives you a discount if you have an approved alarm installed.

At least in my jurisdiction (Norway), you install a car alarm because your insurance company still gives you a discount if you have an approved alarm installed. The cheapest, approved alarm I could find for my beater (an old Land Cruiser) was approx. $200. That translated into a $80/annum discount. Additionally, I leave the doors unlocked to save any would-be thief the trouble of breaking a window to gain entry. There's nothing valuable inside, anyway.