Why is negotiating the scope of a project important?

It is so important to identify and define all the deliverables and the work you will be required to perform to produce them to include what will be included and what will not be included (out of scope). The scope is the foundation of all the project work and ultimately to customer satisfaction, the company getting paid and perhaps for follow-on work. Misunderstanding at the scope phase often means problems later on in the project, which have both time, cost and schedule impact not just on this project, but often across the whole company portfolio of projects.