The master story list is a compilation of statements such as "As a(n) <role>, I want <action>, so that <benefit or result>. The benefit or result comes from the business value model. This compilation of stories helps to further define the scope and thereby improve estimates.
Agile is really nothing more than a repackaging and renaming of older concepts in use for decades. Those of you with a traditional development background may recognize these statements of the "master story list" as use cases and/or context diagrams. They are useful tools to communicate to the project sponsor and other stakeholders. They also help the engineering team in developing the design and by the project manager in working with the team to determine where the key risks are.