For poor people whose ability to live depends on having a car, car alarms must be at least sort of useful to know if your car is being stolen at night. I’m sure they’re just a noisy inconvenience to the wealthy though.
Usually when they are going off in my neighborhood the car is streetparked and that's how they end up getting triggered (sometimes a loud motorcycle can even do it). So the owner could be like well over a 10 minute walk away and out of earshot anyhow. As a result most of the time when I hear a car alarm it rings until the thing shuts itself off automatically to save the car battery.
Hard to prove empirically either way, I think. Even if they do, it's then arguable if this is worth the noise pollution etc. Probably depends on the circumstances in any case, so any empirical result would be too specific to be of any use. Science!
A thief can disable your blaring car alarm in like 30 seconds. Enough time for anyone listening to go "wow someone must have hit the panic button in their pocket" and move on with their life.