The problem with OpenCart is, that during the checkout no check for stock is made.
Therefore it can happen, that a customer orders some pieces from a product on stock.
Afterards he goes to the checkout.
In the meantime another customer oders the same product, but the stock is now empty or low.
Both customers will see that fact only at the checkout process when they want to buy.
This extension checks every time a product is ordered if enough on stock - and adjusts automatically the basket.