Lack of formal documented policies

What is the risk of not documenting formal procedures for basic systems administration? I have a systems admin who says the amount of day-to-day tasks they’d need to document would be never ending. But our external auditors for example made a recommendation that, per quarter, “a quarterly assessment should take place to verify that members of privileged security groups (such as domain admins) still have a valid business need”, which is fair enough, and a good idea to do.

But they say unless the procedure is documented it’s not an effective process? Why so? If an admin does do these checks whether or not the process to do so is documented somewhere, if the work is being done, what’s the risk? Why is the documentation so vital? Or is it? There are other issues, such as backup restore testing, i.e. test a restore once per quarter, change control procedures etc etc. Many of these tasks will be being done but the process to do them is not always documented. But without documenting it, but still physically doing the work, what is the issue.