You are correct, I was a bit slow to the plate on that one.

The DOJ wants the memo because it accuses them of criminality, if they know what the allegations are they know what to burn and I'm sure they will give a courtesy copy to the FBI, so they can CYA themselves.

More importantly Memos like this happen because of "whistleblowers" who have confided in someone else and provided evidence, and these "whistleblowers" have been disloyal and must be found out. The "memo" would likely lead to the source of the information.