Provide the delivery team with good feedback and useful info for future engagements
Document project-specific issues that may be relevant to tech support after hand-off
Increase customer satisfaction and provide the customer with useful project information as they move into the post-implementation support mode
I discussed this to some degree in the methodology article Project Phase 8 – Post-Deployment. A good Project Manager should be documenting potential Lessons Learned throughout the project, but at the very least many things that are included throughout the engagement on the issues and risks lists will eventually be discussed during a more formal Lessons Learned review.