Having history for the project plan and custom fields
Be able to rollback to old version. or compare new schedule plan with old one.
Also view plan actions log, like who saved or published new version.
Same idea also for custom fields
Malyssa Brannon commented
Two other similar asks right here on the page, but yes, we need to be able to roll back just the same as we can with Word, Excel, other Office and SharePoint documents all have versioning, but yet none for the most important documents of all? It makes no sense!