Add-lib access to oyster shell grit will ensure chickens get enough calcium. An important aspect in the diet to form a strong egg shell which is made up of largely of calcium and healthy bones. Soft shells are poor quality shells and a sign of calcium deficiency.
Chickens that lay a larger number of eggs, especially high production hens will always need a higher level of calcium. Many feeds will not supply enough so providing a bowl or tray in the pen or simply scattering over the ground will give access whenever needed.
Once eaten, grit will be in the crop for a length of time where it assists the grinding of grains and seed for digesting. Once dissolved it will provide a source of calcium.
Grit does not harm chickens even when they appear to have healthy shells.
You will be able to tell if your chickens require extra calcium in their diet as the eggs they lay will have poor quality shells. Even if the shells are healthy, extra grit in the diet will not harm the birds.