The only way to avoid problems happening yet again in the future is to carefully consider what went wrong this time (and why), and decide what can be done differently next time to avoid those problems. Likewise, teams can help others duplicate their successes only if they
can express concretely what went well on a project, and why. Lessons learned processes help identify and transfer such recommendations forward from one team to another. The specific lessons and recommendations generated via lessons learned surveys and meetings yield concrete actions for other teams.
This process is also useful within a particular project. Some teams have interim lessons learned meetings during or at the end of each major project phase, with an eye to recognizing what is working well, or not so well for the team right now, so they can take the chance to adjust. It's also a good idea to capture lessons periodically during a longer project, before the team moves on and forgets the fine points of lessons learned so far.
Either way, the goal is to build upon the successes and lessons of prior teams, and—within a project—to reflect on what is working, and where changes could improve the team's work and the overall project.
See our Lessons Learned Survey, our Lessons Learned Meeting Agenda, and our Lessons Learned Meeting Report for tools and guidance for doing lessons learned activities.