10/30/2009 – What usually happens is that extra forces help the besieging army storm fortresses. But there is also the opposite case, when the presence of extra material actually saves the defending side, as in the present situation. If he did not have his rook, Black would be totally lost in the duel between White's remaining rook and the black bishop. But White simply cannot force the exchange of rooks:
Analysis Spoelman-Najer