These organs are dropped by the Ubers as part of the Infernal Machine, and you need them to create the Hellfire Ring and Hellfire Amulet. This is reminiscent of the drop odds back in D3v, when each level of Monster Power added 10% up to a maximum of 100% drop rate on MP10.
Unfortunately, we don’t know the actual numbers for T1-T5, so it’s not clear just how much you gain by pushing the difficulty each notch higher. Possibly Wyatt Cheng will twitter-swoop in to add details, as he did a few days ago re: the unstated buff to legendary items dropping from Horadric Caches?
We fired him a tweet, hoping to create just that scenario. Everyone go RT it so he’ll feel guilty. Use sarcasm if possible; Wyatt loves that.
— Diablo II Net (@DiabloIINet) September 10, 2014