The answer to this question is already partially in the question: Picasso wanted to show show the horrors of the war, as experienced by the civil population, to the world. He wanted to show that war has devastating effects and by this turn people's opinions against the war.
The bombing was also done by the fascists so the painting can be seen as a symbol for the fights against the fascists.
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Shortages affect people in a variety of ways. If there is a shortage of food many people can die from starvation. If the is a shortage of water (a drought) many people can not only die but no one will be able to bathe and the animals can suffer from dehydration. On the other hand, a surplus is entirely different.
