The key takeaway on this "master" business is the very real, and very true, point that Trump makes -
The DOJ and the Feebs cannot be trusted with confidential and private material (protected under lawyer-client practices). Furthermore, any chain of custody is irretrievably broken at this point given the absolutely shoddy way the Feebs documented what they stole during the raid. What kind of documents were planted? The Feebs couldn't answer that question with absolute certainty since they failed to record a summation of EVERY DOCUMENT they removed from MAL.
The appointed master would render a complete, impartial view of the records that the Feebs stole and that would comprise Trump's defense when he is indicted -- as he surely will be.
DOJ and the Feebs didn't do all this by accident. No, it was a fishing expedition and they believe they "caught" something. The master will ultimately convince a grand jury, ideally before the inevitable indictment, that this entire pile of stinking offal is part and parcel of yet another attempt by the Deep State to keep Trump out of the WH again.