Without teeth, grit, for all birds, gets lodged in the gizzard so they can grind up their food. It needs to be the appropriate size. Too small and it passes right through.
Theoretically, if they only eat commercial feed they don't need it because the grains contained therein have already been ground. This is a yes and no. The grains aren't completely ground and the chickens can benefit from grit to break them down further. Also, regardless of what they're eating, grit helps develop the gizzard.
For that reason, all birds should have grit.
Free ranging birds can find their own grit IF the appropriate sized sharp stones are available in the forage area.