What's a billing account on an opportunity?

A billing account on an opportunity is the customer's financial account to be used for the transaction. This field only displays the billing accounts if you have an Account associated with the opportunity.

The billing account won't display any values if you only have a Contact associated with the opportunity. For example, if you select the business-to-consumer (B2C) contact on the Account, then the billing account field displays the corresponding billing account but no value is shown if you select the normal contact on the Primary Contact field.