I think it depends on how much time they spend outside where there is dirt, sand, gravel, etc.
I've had chickens for years, and have never purchased grit. They do fine just pecking the ground in the yard and eating what they find. They spend the day in a fenced-in yard, except when there's snow. Even in winter they go out as soon as the sun melts snow on the path I shovel to get to their barn.
However, I am thinking of putting a largish container of dirt in their coop this winter, as I have changed the location of the pop door. It will now be on the shady side of the building, so not as much path melting. I'm not sure they will want to venture out as much.
Does anyone have experience with putting a container of grit in the barn in the winter? Can I just use regular dirt? Or maybe sand?