A lot of good answers, as there are a lot of reasons carry large inventory. I'll add.

Grocery stores carry a large range of items to draw customers in.

Think of that one product you love, that is hard to find but you know the one store in your area that carries it. They know you buy it every now and then, and they know it's not a common item.

Carrying that one item (and the other less common items people like) brings you into the store and you'll most likely buy more while you are there.