The business requirements document should accurately, and in detail, describe the purpose of the project. It states what is needed to achieve that goal i.e. what is in-scope, what is out-of-scope, any assumptions that have been made, any constraints that have been imposed and expected timescales.
The document will form the definitive description of the aims of the project and, as such, can be used to manage the expectations of the stakeholders. It will also include acceptance criteria that will ultimately be used to judge whether the project was a success.