What do you consider to be the most important benefit of carrying out a project post Implementation Review?

the important is no back job if properly coordinated

I think the most important benefit of project post Implementation review is the lessons learnt in the entire project duration. Specially on the part what went wrong in the project and necessary inputs to be implemented in the future projects. This will give you immense help as far as project duration and project costing part surely.

There could be some listed down benefits:

Learning from the project

Benefits to the organization from the project

whether object if the project was gained. etc.