The very minimum thing that your shopping cart software should do is track the inventory that you have versus what was sold. Zippycart.com puts it this way,
What good is your site if you can’t accurately track your inventory, or collect payments through a shopping cart? At a minimum, shopping cart solutions provide powerful inventory tracking and payment management functionality, allowing you to better report on and manage your product.
All that the software is doing here is telling you what you have in stock, what you have sold, and the price that you got for it. Just knowing these few basic things will come in handy when you are doing your accounting.