Why should we do requirement-based testing?

To receive the high-level of requirements.
Test of the requirements is achieved How do we best formalize the requirement

The requirements-based testing (RBT) process addresses two major issues: first, validating that the requirements are correct, complete, unambiguous, and logically consistent; and second, designing a necessary and sufficient (from a black box perspective) set of test cases from those requirements to ensure that the design and code fully meet those requirements.
  The overall RBT strategy is to integrate testing throughout the development life cycle and focus on the quality of the Requirements Specification.
This leads to early defect detection which has been shown to be much less expensive than finding defects during integration testing or later.
The RBT process also has a focus on defect prevention, not just defect detection.

The UAT phase is very important to see if the implementation has been done as per the requirements which were documented. But yes, before UAT itself, it's always great to have show and tell sessions so that the business teams can find out issues much before UAT begins. 

 

Additionally requirements based testing ensures that all the test scripts are thoroughly validated before go - live.  

we are doing requirements based testing while we design test cases to insure that test cases cover all requirements provided buy the user, try all scenarios that user can make as a black box user, ensure that we are delivering the right product