The purpose of the plans theme in PRINCE2 is to facilitate the management and control of the project by establishing whether the project's targets are achievable. This involves setting out how and when the project's objectives are to be achieved by defining the necessary resources, activities, timescales, and costs.
References: PRINCE2 Foundation Handbook, Plans Theme section.

The Project Product Description in PRINCE2 should clearly define acceptance methods to ensure that there is a concrete plan for demonstrating that the final project product meets the customer's quality expectations.
This is crucial for aligning the project's outputs with the customer's needs and expectations. While it is part of broader quality control and assurance mechanisms, the specific role of defining acceptance methods is to make clear how compliance with quality expectations will be verified.References: PRINCE2 7 Foundation study guide and documents.

Change management is important in a project because it provides structured processes and management products to manage and implement changes effectively. This ensures that all changes are assessed, approved, managed, and documented properly, helping the project adapt to changes while maintaining control and alignment with project objectives. References: PRINCE2 Foundation official resources and training on change management.