Why are lesson- learned report important in IT project management important?

Lesson-learned report are very important since whenever project completed we may use it as a guidance for improvement to existing project and it can be used for incoming project/s. It is also vital to the project's team to not duplicate the same mistakes that have been existed in previous project thus make it smooth in next new project/s. Furthermore, the content of the report itself can be reduced from time to time as it is referred to previous project compared to the new one.