They are questioning these witnesses under oath so if evidence turns up (or they already have it and are saving it for the right moment), they can hang a charge of perjury over the witness's heads and see if that makes them more cooperative. Little by little they exploit the cracks in the administrations stonewall until the bring it all down.
This is the GOP. they have a history of their supporters believing that such is the case only to then cave, choke and otherwise return the hammer of great and mighty justice to Home depot for a ‘partial’ refund rather than use it.
Since no investigation thusfar has done anything other than protect democrats from jail and act as a pressure relief valve for angry conservatives I have little faith in some cunning plan afoot.