You need items because your customer places an order for those items or products that you either build, manufacture, assemble, or buy to be resold with value-added components. These may also include services such as warranties and extended warranties.
In any of these cases these (and other related items) need to be defined and available for use when processing an order.
Chapter 4, Inventory described how inventory works and how items are defined for transacting in either Purchasing or Order Management.
In Purchasing, you can have a Purchase Order that does not have an item. These are referred to as expense Purchase Orders. Examples of these could be office supplies, furniture, and so on.
In the case of a sales order you must have an item. It is also a must to have specific attributes checked (or enabled) to be transactable in Order Management.
The following is a screenshot of the tab where order related attributes are managed. The screenshot shows a few key attributes that need to be checked...