If you live in the rest of the United States, which is spread out and some quite rural, a car alarm can quickly notify you someone is messing with your car.
My town has a population a little over 300 people, we have one resident that is... not well... that rides his bicycle around even int he dead of winter and pounds on peoples doors screaming that the government is coming (he did this Saturday to a woman who finally called the county sheriff), a few weeks ago he pulled a battery out of a forklift at a local business's dock and proceeded to smash it on security camera, rides up to people that are out and about screaming nonsense at them, etc. Then factor in "kids being kids", the people that were spotlight poaching deer from the highway - shooting in the direction of houses - a month or so ago, etc and car alarms are just one of those handful of things that actually do come in handy in some places.
Even in high school in the early 2000s I had someone break into my car in the driveway at night in a cookie-cutter edition and go through my car. With an alarm we'd have known they were doing it.
Several years later I had someone crawl under my truck, cut the fuel line, and after taking all the gas they could get using a piece of tape to try and hold it back in place (I actually had this in my Facebook memories yesterday), a car alarm may have alerted me to that.
Then my mother's boyfriend passed away around the same time. Someone flat out stole his decades old beater out of the driveway at night, again an alarm would have alerted them to it. Unfortunately for the thief the brakes barely worked and they didn't get very far before plowing into another car and taking off on foot.
Car alarms definitely serve a purpose, and I don't need Popular Mechanics to justify them or not.