Good specs do. You might write a spec that describes the functional requirements of a claims processing system, because your current system is 20 years old and

Good specs do. You might write a spec that describes the functional requirements of a claims processing system, because your current system is 20 years old and can't cheaply be modified to comply with legislative changes. An obvious "business concern" is that any replacement system will accommodate future legislative changes. This has nothing to do with the functional (user) requirements (form design and submission process), and yet might dramatically change how you design and write such a system. DRM, or rather the encryption scheme that makes it DRM, is much the same in that a design that supports it could similarly support related but more benign capabilities, such as compression or translation. That too will influence an implementation, and makes such a non-functional requirement as important as a functional requirement.