A well-written description of any project makes it possible for the intended audience (e.g., the sponsor, the customer) to understand the concept and context of the proposed project and to realize whether to approve and finance the project.
Writing a project description document is a general task of the project manager or deputy, who is supposed to correctly and comprehensively identify the idea, goals, background, approach, outcomes, and other data.
Furthermore, the document should define the project as a worthwhile, economically practical, and reasonable endeavor to convince the sponsor of the need to make the necessary investments.