In addition to tracking costs and quantities for traditional stock-type inventory items, using inventory items makes entering transactions easier. You can set up all the known constants for any item that you might include on an invoice. That information will automatically appear, once you select the item code. This saves data-entry time.
For example, you can set up freight charges as an inventory item. Most likely this would be a non-stock item, because you would want to track default accounts as well as a description and a unit price (which can be changed on the invoice). The item class you choose depends on how much information is constant for the "item."
Be sure to enter items such as freight as non-commission. (That is, don't select the Subject to Commission check box on the Maintain Inventory items window.) Then, sales representatives won't receive commission on these sales.