I believe it is what the previous two posters said.
A prosecution witness provided testimony that undermined the case and now the witness needs to be rebutted by the most venal vengeful self-serving witness/attorney they can find - Cohen.
(Someone told the truth and it needs to be slapped down by the most venal vengeful tawdry prosecution witness available; Cohen.)
Anyways...we'll never know...cuz GJ info is top secret...unless the Dems are around.