User stories are accumulated in a 'back log', a list of stories to be prioritized in 'back log grooming' meetings by Product Management, then estimated by the development team, and finally assigned to a sprint where they are actually implemented by the team. User stories are the fuel that runs agile, without user stories there is nothing for Product Managers to groom, nothing for developers and QA to estimate and nothing for the team to implement and deliver, there is no fuel for the engine. Many times user stories become the Governor of the velocity of the team, or how fast the team can produce, without enough user stories the velocity is less than optimal or in some cases I have seen zero, yes developers actually 'sitting on their hands' doing nothing.
So if you are embarking on or are implementing an Agile development process pay close attention to the fuel of your scrum, the user stories. Who is writing them, refining them, breaking them up into smaller stories that can be more accurately estimated? This can be your weakest link.