Whatever the end result of the project, it will involve some type of change within a business. The change could be a modification to the existing status quo or it could be introducing something completely new, so change management is also an element of project management.
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.