It’s to protect from falling debris from the ceiling. How much it helps is debatable but it’s best they have there in school. More effective on traditional bombing than nukes
In Finland we have bomb shelters everywhere, it’s arguably more effective
Edit: I’m too drunk to write coherent sentences
In WW1 armies learned helmets were a good idea when artillery kicked up big chunks of debris killing unlucky soldiers when it rained down on them. Ballistic protection was an afterthought that came along later.
So yeah better than nothing I guess, same with tornado drills our schools have sometimes
They almost stopped using helmets again, too. The number of head injuries skyrocketed. Thankfully, someone pointed out to command that the helmets weren’t causing the injuries, but converting fatalities into injuries. They hadn’t been recording head injuries on corpses.
