For some people, Account Details are all they need to fill in on a Bill or PO to satisfy their business. They fill in the account to set the Bill or PO against then use a description and that is all they need.
Other people either need to fill out the Items Detail section or need to to work with inventory. Filling out this section will give you more detail in what you are purchasing or service you are paying for.
Either way, you must fill out the Account Details section but the Items Details section is optional.