This field is optional. One of the best uses of this field is in the form the users see at checkout, and specifically when showing the details of an item on the checkout page, via the template named "view_cart_item.php". If you have a rather long description of a product, you can add a brief description in this field for the purpose of showing the shorter description wherever you need it. If you use the $Brief_Description variable in a template, and the field has not been filled out, it will be replaced by the contents of the "Description" field.