There had been famine in France and the people didn't have flour to make bread even. As you learn about history, you will discover that bread was an important thread throughout the times. If you look at the Depression of the United Stats you will see mention over and over about the bread lines. It has been common practice throught the history of human kind at one time or another to survive on only bread and water.
If memory serves me not only was the Bastille a prison, I believe it is where they stored the grain.
It is said that when Marie Antoinette was pleaded with to give the people grain to make bread that she said "Let them eat cake" Unfortunately you need grain to make cake too. Shortly thereafter she lost her head.