The necessity to build bunkers against air raid started with the development of aircrafts good enough to transport a heavy load of bombs over an appropriate distance. This happened first during World War II. The aircraft of World War I were still primarily used for air reconnaissance and for fights one on one.
As the invention of heavy bombers were a general invention of numerous countries, they also tried to build bunkers for defense. They should house vital infrastructure and save it from air raids. Some bunkers were military bunkers for the General Staff, others were used to store valuables or art. Some bunkers were intended for the safety of high personalities, others for organizing a battle. And very common were bunkers used for weapon and armament factories.
But most of the bunkers were used for people, they had to run to the next bunker when the siren signaled air raid alarm. To find them easily, the bunkers were signposted. Many houses simply had a cellar renamed bunker, but some cities with vast underground cellar systems or mine shafts had very good air raid shelters. Other cities created concrete bunkers, which were then covered by a few meters of soil. Unfortunately many of those bunkers were rather unsafe. If the house above was destroyed the entrances were often covered by piles of debris. The people could not leave them anymore and so many had to die of suffocation. Or they collapsed from the weight.
All those underground shelters were intended to seat the people for a few hours while bombs were falling. The main purpose was to resist the explosions and the dangerous fragments. So it was nt necessary to stay inside the bunker after the end of the air raid. As a result they were equipped only with the minimum. People had packed suitcases with some basic things which they took with them to the bunker. Probably something to eat and drink, probably a book or some toys for the children.