Read somewhere that was also part of the effort to get Spiro the Bag Man Agnew out of office. If I recall, there was some speculation that there were those who saw the prospect of Nixon either being impeached and removed or resigning and Agnew moving to the Oval Office to be even more grim than Nixon remaining in power, since bags of kick-back cash lying about on the desk of the President might not be a good look. And Nixon, knowing this, wasn't above using it as leverage. So, the memo also had the purpose of reassuring him (Tricky Dick) that the Justice Department wouldn't come gunning for him, and thus smooth the way for Agnew's departure.

Don't know if that's completely true, but it's an interesting theory.

Spiro Agnew...now, you talk about a guy that needed a little prosecutin'...