You know, or if you’ve gone somewhere with the full expectation of being long gone by dark, yet are detained because of some unforeseen instance. Hell, maybe even an emergency. Those things never happened to well-prepared.
I set my alarm when the car is parked in my driveway, when I am out and about doing errands, or if I am going to be close at hand (a friend’s house, work, etc). Basically, if I am close enough to be able to do something about it and quickly, it gets set. So, 99% of the time, my car alarm is set.
If it is going to be on its own for any length of time, I don’t bother. I parked my car for two weeks while I was in the UK and just locked it. It was fine. I would have hated to have come back to find that the alarm had been tripped and I was thousands of kilometres away with the fob.
There is someone in my neighbourhood, I don’t know who, whose car alarm goes off in the middle of the night about twice a year. For a very long time. At least 20 minutes. I suspect it is not in fact a neighbour, but a guest of a neighbour. An idiot guest. Who should be banned from polite society. And perhaps sent to the special hell.
But to answer the OP, a car alarm draws my attention long enough to see if there is any suspicious activity. I have yet to see any, so I can’t say for certain how I would respond. Probably something with shouting. Who am I kidding? It would start as a polite inquiry as to whether everything was okay. I think they still serve a purpose.
While we’re on the subject of car alarms, there is at least one benefit-
Most major insurance companies offer a discount of 5-10% off your Comprehensive coverage premium for having one…but they rarely give it to you automatically. It makes no sense to me, as an insurance agent, but most insurance company rating systems still require an agent to manually select that a vehicle has ABS!