Business requirements are statements about the outcome the client is looking to achieve. Without these statements, the engagement has no basis for determining if it is a success or a failure.
Once the business requirements are created and approved they form the basis for measuring on an ongoing basis.
Solution requirements are traced back to the business requirements so a discussion on how the business requirements are satisfied by the solution requirements can be had.
UAT scenarios are traced back to solution requirements and then to business requirements to understand what outcome a UAT scenario is testing.
Cost analysis is based on the business requirements and the revenue created, the expenses saved, or assets built.
Business requirements become the barometer for evaluating all decisions made on the engagement.