I think the OP did answer why she thought the vet was recommending Heiro. He heard it can do good things. (See post 8). If he has more of a reason it sounds like he did not communicate that reason to the OP.
Therefore I am going to ask a few questions that I think people are trying to get answered to help the OP:
Is your horse still having IR symptoms with the Quiessence and the Vitamin E? Or does she have a condition that might be caused/contributed to by IR. If so, what are the symptoms or condition? Or did the vet as part of a routine vet visit mention that Heiro works well for IR horses so you should add that to the diet?
I think people are trying to figure out what the vet is hoping to solve by recommending it or if it was suggestion since she is IR and OP should use it even if the IR is currently controlled.