The calcium has to come from somewhere. Oyster shell is cheap, but if you're feeding a layer ration they may get enough from just that. I usually supplement with shell though as I find the layer ration usually has the mininum amount of calcium they can get away with. If your birds free range that means they may not get quite enough or they may over eat the layer ration to get the calcium they need even if they don't need the protein. Depends on the bird.
But you can also use egg shells, clam shells, feed grade ground limestone, and probably several other sources that haven't occurred to me. When a pullet first starts to lay the calcium initially comes out of their bones, but that can only last so long before she runs out. You really want to give her a calcium source before she reaches that point.
FWIW, except for those birds that aren't eating oyster shell I don't feed grit to anything and there is no gravel on the place. The oyster shell will work as grit though it may not last as long as harder stuff but since they need to eat it regularly that's OK. A chicken's gizzard does not care if it's granite, oyster shell, or a diamond ring it'll use whatever is available to grind stuff up.
.....Alan.