Not 100% sure on the question but I'll answer it both ways I'm reading it.So why do people do this if you still have to supply them with Oyster Shells.
1. Why use eggshell? Because some birds prefer crushed eggshell over oyster (I mix both in the "oyster" bowl so they just kinda take whatever is in front of them). And it helps stretch your dollar - the oyster shell I buy isn't the cheapest so adding eggshells in extends out how long a bag will last, while still meeting their calcium needs.
2. Why bother with oyster shell? Because especially if you don't use layer feed, there's not enough calcium in the feed itself to meet a laying hen's needs. Nor in eggshells, as those draw from the calcium in their diet, so as you can imaging as they continue to produce eggs they're using up more calcium than available in non-layer feed + eggshell. In your case assuming you only feed layer feed, that's adding extra calcium back into their system so supplementing with mostly eggshells might be enough, depending on the needs of your flock.
