Throwing rocks at cars

has to be one of the most stupid, idiotic, insane, and dangerous "pastimes" out. I simply cannot believe anyone would actually find it "fun" to do this.
Yesterday I went to deliver a knitted garment I had repaired for someone. I had rung him first to see if he was home and he said,
     "Yes, thanks. Come on over and have a look at this. You won't believe it."
His neighbours have a new-to-them car. It was one of those "demonstration" models - a car at a reduced price because there were a few kilometres on it but in otherwise pristine condition. Well, it was in pristine condition until yesterday.
When I turned up my elderly friend and his neighbour were still standing there staring at a large dent in the back and a shattered rear window. His wife was sitting on the steps looking white and tearful.
Her husband said tersely, "Rock. A.... was driving."
I was too shocked to say anything. I just went and sat next to her and she burst into tears again. She isn't somebody I really know at all. I doubt we have said hello more than three times. Cautiously I put my hand out and she grabbed it so hard it hurt and then she buried her head into my chest and howled. It was awful. The two males stood there looking helpless so I mouthed "cup of tea" and at least my elderly friend had the sense to realise what I was trying to get across. He is rather deaf and does depend partly on being able to read lips.
Her husband went off to make tea. She calmed down a bit and started to apologise. I told her I'd feel the same way, probably worse. Having a good howl probably helped a little but it will take more than that to get over the incident.
Had they called the police? No. I told them they must. Yes, of course. The husband went to make the call. About twenty minutes later a patrol car arrived. I left them to it. 
I started to pedal home - and then realised that my elderly friend's vest was still in the bicycle basket. I went back and delivered it.
We both agreed there is no chance of finding out who threw the rock. That nobody has been killed in the recent spate of such incidents is extraordinary.
Just what do people get out of doing this?