Hi kvp
Well, it can have many uses. For example, if you are a shop owner that has an online and a physical store, perhaps the total stock that you have of a product is 10, but you know that 2 of these 10 items are for selling in the physical store not in the online one, so when the stock reaches the quantity of two, the item will be displayed as out of stock.
You can always put this value in 0 if this behavior is not suitable for your business.
Best Regards,
Andrea.