Chickens eat grit and pebbles along with their food so they can grind up seeds, grain and other hard food in their gizzard. They need these otherwise inedible items to help them grind hard foods like grain because they have no ability to chew. The grit and pebbles eventually are passed through their digestive system as part of waste products.
The gizzard is a strong muscled organ like a couple of tiny catcher's mitts facing one another. It has a tough vinyl-like sheath on the inside that separates the muscle from the cavity where food is processed. The muscles roll and crush food inside of this tough sheath before it's sent into the intestinal tract.
That's why chicken gizzards are always cut open when you buy them in the store...the tough sheath needs to be removed during processing because there is no way you could chew this tough rubbery material.
While butchering a bird, if you feel the gizzard a bit before cleaning it out, you can sense the grit and rock material inside it. As odd as it seems, inedible materials are essential for chicken digestion.