When doing system implementations, you’ll need to have both functional and non-functional requirements specified – in detail.
It may well be that all requested system functionality has been implemented as specified, but system response times are unacceptable slow. Or the system can only comprehend 100 concurrent users, while 300 concurrent users are expected.
Or the system response time is easily met, while some basic calculations or presentation in the Graphical User Interface (GUI) are missing.
In this way, both functional and non-functional requirements are necessary and supplement each other.