I have changed your question to refer to World War I rather than World War II. Artillery bombardments were much more a part of the earlier war. In addition, your other two questions, about Spanish influenza and “shell shock,” clearly belong to WWI.
Artillery bombardments were a tactic that was used quite often in World War I. They were used because of the nature of the trench warfare that typified combat on the Western Front in that war. In this war, soldiers spent most of their time dug in to a series of very fortified trenches. Each side had no way to advance other than frontal assaults on the enemy trench systems. These frontal assaults were very costly in terms of casualties because they involved men attacking entrenched opponents (who had such things as machine guns) across open ground.
It was for this reason that artillery bombardments or barrages were used. The idea was that the artillery would kill so many of the enemies and damage their trench systems so badly that an attack would be able to overrun the enemy positions. In these barrages, huge numbers of shells were fired. These caused casualties in many ways. They killed people with direct hits or near misses that sprayed shrapnel. They killed people by caving trenches in on them. They also disabled men by causing “shell shock.”