In the project charter, the Purpose best describes the project provides justification for it.
The purpose is a detailed document which covers the following areas:
1. Reason to undertake the project
2. Project benefits
3. Objectives of the project
4. Problems and constraints of the project
5. Main stakeholders
6. Budget and budget spending authorities
7. A Risk matrix
8. Solutions
9. Items of scope
Overall, the document provides a detailed picture to a project manager.