Chickens do not have teeth.Ok, but why do I need it?
They swallow little rocks (grit) which rub together in their gizzard to grind their food.
Purchased chick starter has already been ground up, so the chicks do not need grit when that is the only thing they eat. But if you give them grass and other things, they do need grit in their gizzards to grind it up.
If you give them a shovelful of dirt, they may pick enough little rocks out of that.
But grit is fairly cheap, so it is common to just buy it and provide some to the chicks. (You can put it in a container, or sprinkle it on the ground, or sprinkle a small amount on their feed-- how you serve it is not a big deal.)