As Dershowitz has said, the Mueller report is from the prosecutor’s viewpoint and that the defense (Trump’s team) should have the ability to view the report and issue a rebuttal at the same time the Mueller report is released.
Plus Barr may want to give a “last chance” to assert executive privilege, or he was told that the president wants his team to do an executive privilege review - and that means the material has to be known/shown.