The purpose of a lessons learned document for a project or company is to capture the lessons learned from our experience throughout a specific journey in a formal document so that other project managers on similar future projects can learn from that. This document can also be used as part of new project planning for similar projects to understand what problems occurred and how those problems were handled and may be avoided in the future.
Simple lessons learned template serves as an important tool for use by project managers within a company who are assigned similar projects. The document does not only state what went wrong during a project and suggest how to avoid similar occurrences in other projects, but it also describes what went well and how similar projects may benefit from that information.
Capturing lessons learned is a continuous process, not to be left at the end of a project. It can be as informal or formal as you would like. Typically, after any presentation or meeting, it is good to capture lessons learned so that important feedback and shared understanding is well documented and guides the future course of projects.