When defining the requirement, it is also the time to define the acceptance criteria. The requirement is about "I want the system to be able to do...". When I started writing requirements, I also try to start with the sentence "You should be able to ...".
For example, the requirement is "You should be able use the same password to login to BI as the password used to login to Apps". The testing scenario will be:
Setup a User and define the password in Apps
Try to use the user and use the password to login to BI.
More testing scenarios:
Try to alter the password in BI
You should not be allowed to do so, or
you should be able to use the new password to login to Apps.
The Use Case method is forcing people who write the requirement to think like a QA for preparing the test case. The test cases becomes use cases, which represent the requirements.