There's going to be a lot of badgering from both sides.
If he sticks to his statement he won't offer any new insights.
Mueller timed it so Barr was out of the country and blindsided but the press conference in an official DoJ conf room with the DOJ crest in the background to lend undeserved legitimacy to Mueller betrayal of Barr, an old friend.
Barr is not going to take this lightly and he holds all the cards - including unrestricted authorization from the President to declassify documents incriminating to the Mueller investigation
Barr is a lot smarter than Mueller so expect him to put a shot across Mueller's bow. Not sure if it will be preemptive before the hearing or damning after the fact - or a bit of both, but Mueller is going to be toast by the time Barr is done with him