The reason they may need it is that some flocks that spend most of the day foraging, or are given too many treats, can develop a calcium deficiency, particularly if they are high production layers. If they're eating too many treats, they won't get all the calcium they need from their feed. It would be like feeding your kids nothing but fries for every meal. They may be getting calories; they may be loving the food... but they need more nutrition than they're getting in order to stay healthy.
Laying hens who aren't getting enough calcium can begin producing weak or irregularly shaped eggs--even eggs with no shells!
This issue can also cause slow laying or even behavior problems like egg-eating, because your layers are so desperate for the dietary calcium they need.
So, to avoid all these problems from the get-go, we recommend offering a source of calcium like oyster shell, free choice so they can take just as much as they want. We recommend you cut down on the treats, too! MOST of their nutrition should be coming from their nutritionally balanced feed, rather than leftover spaghetti, right?
There are a number of reasons your chickens might be producing weak eggs---or no eggs---though, so if you suspect there's something more to it than a lack of calcium, have a look at this list of other possible reasons your chickens aren't laying well.
Do keep in mind that oyster shell isn't necessary for pullets that haven't started laying yet.
Some people prefer to save their egg shells and offer those back to the hens--that works, too! However, please remember that a hens' own shells will not provide her enough calcium if she doesn't have another supplement, since she also needs calcium for other body processes. If you choose to offer eggshells back to your hens, be sure to bake them first (350* for 10 minutes or so) to kill any bacteria on the shells, and make sure they are crushed finely enough that they don't look like eggs... or else you may be providing calcium and ALSO teaching your hens to eat their own fresh eggs.