No matter how hard you try to keep OmniFocus up to date, eventually there will be a divergence between OmniFocus and reality.
You'll complete tasks but forget to mark them as completed in OmniFocus. You'll intend to do things without creating tasks or projects in OmniFocus for them. Due dates will change. Projects will stop being relevant.
To make sure you stay on top of your projects, you need OmniFocus to accurately and fully reflect the reality of all of your projects.
And that's what the periodic review is for.