There is a non negligible chance that Trump regains the Presidency. If he does, he might blather on about his supposed immunity because "the supreme court, that supreme law of the land, holds our nation together folks. Very good justices. I appointed three of them you know... they didn't rule. That court in the Swamp... they ruled.. unjustly I might add... you know my wife..."
By granting cert and ruling they will take that risk off the table. Then the buffoon will go to trial and he will likely abscond. Fin.
If he regains the presidency, he will pardon himself.