Business Areas correspond to specific business segments of a company, and may cut across different Company Codes (for example, product lines). They can also represent different responsibility areas (for example, branch units). The Business Areas are optional in SAP.
The financial statements drawn per business area are for internal reporting purposes. You need to put a check in the check box in the configuration for the company for which you want to enable business area financial statements.
When transactions are posted in FI, you have the option of assigning the same to a Business Area so that the values are properly captured for internal financial statements.
The Business Area can also be derived from other account assignments, for example, cost center. But to do this, you need to define the Business Area in the master record of that particular cost center.