in which we consider the essential contribution of requirements
Requirements are not really about requirements.
Requirements are what the software product, or hardware product, or service, or whatever you intend to build, is meant to do and to be. Requirements exist whether you discover them or not, and whether you write them down or not. Obviously, your product will never be right unless it conforms to the requirements, so in this way you can think of the requirements as some kind of natural law, and it is up to you to discover them.
That said, the requirements activity is not principally about writing a requirements document. Instead, it focuses on understanding a business problem and providing a solution ...