There is no nourishment in bits of gravel. But if a chicken did not swallow a few sharp little stones she would get no nourishment from her real food. She has no teeth to chew and every morsel is swallowed down whole. The grains and grubs go down to wait in a storage pouch called her crop.
Later, food from the crop goes down to the gizzard, which is a special stomach. It is made of very thick, strong muscle and shaped like a tough little fingerless fist. Since the food is not chewed, the gizzard must mash it to a pulp so that it can mix with digestive juices. This job is easier if the tough little fist also holds some bits of gravel.