great question. In my answer I want to give a stylized example to showcase why the various concepts of scenario, UR, FR, etc. exist.
Before one designs (builds) a system, one needs to have a set of requirements. These may not completely restrict the implementation, but specify it enough to fulfill the needs of the imagined end user. Ha! so one needs to imagine a user, and specify her needs.
So here's an example of a functional requirement:
New questions shall immediately become visible to all other users.
The concepts of personas, scenarios, use cases serve as a way of generating such requirements. It's all structured creative work; so there aren't any strict rules about this, but I hope that you'll get the drift.
First, one specifies a persona, like a short CV:
Tom is a senior software engineer that starts to assume managerial roles. In the new role he'll need to acquire lots of new skills on demand ..: Does he typically use the internet via his smartphone etc? What's his technical skill set, his personality? Anything that might be important.
Here comes a scenario with Tom, i.e. a user need derived from his persona:
Tom needs to quickly understand the difference between scenarios and use cases so that he can better communicate with his new manager colleagues. ..more details about Toms circumstances.. . So he visits stack exchange and asks about it .. .
From such a scenario for the persona Tom, we can derive several use cases, for example
Knowledgible users should be able to view and answer new questions as soon as these are posted to the platform.
Again to drive home my main point: personas, scenarios, and use cases can be used to derive and justify functional requirements of a system. Backtracing such a stack will inform you on how the users might be effected when changing a requirement. In medical design, essential requirements trace back to hazardous scenarios; so one knows better not to change these.