Giving a functional spec to developers and testers is not sufficient for creating great software. To a developer, a spec is only the what and not the why. And for a tester, the software requirements specification is neither. Use cases provide the why that explains the intent of the system for the implementation team.
Communicating Intent With Stakeholders
We can build a prototype of what the stakeholders don’t want, and then get feedback and fix it. Or we can review use cases of what we intend to build, confirm that each stakeholder wants it, and build it right the first time.