The Special Master is about "Due Process"

 Or will an ex-president of the United States have equal protection under the law as any other U.S. Citizen?

So, when you are hearing about a Special Master this likely is the reasoning about all this from the judge.

However, it is also a "Can of Worms" not only for the Justice Department but also for Trump and his attorney's. In some ways a special master means (Everyone loses something) in this.

I personally think Trump will incredibly regret asking for a Special Master because it is likely to harm him incredibly and make his likelihood of being indicted about 10 times as likely as it was before for a variety of reasons.

So, Asking for a Special Master might be the worst thing that Trump ever asked for in the long Run through his lawyers.

In other words I presently believe: "Trump is Hoist by his own Petard!"

Translated from ancient French this means "Trump will be blown up by his own hand grenade!"