Lessons learned from each project can be used to improve the next project. Lessons learned can be about things to avoid, things to repeat, good sources. Lessons learned are part of the organizational process assets. Lessons learned can be captured at any point during the project, but especially as you close project or phase.
In order to be most effective lessons learned must be documented and saved where they can be accessed by others. A PMO is one good place to save the lessons for sharing.