Ashfaq,
Practical application of tgt torque IMHO. If you have an OEI at high torque settings, one of the first actions would be to lower collective to restore / maintain rpm.
Lower, but how much, to what setting?
Too less, rpm would drop. Too much, you are not making optimum use of the power available from the good engine.
Target torque gives you a good guideline of min torque to which you can safely lower collective (Min assured torque) and then maybe come up a little, N1, Rrpm and ITT permitting, if the engine is giving a better performance (than the minimum assured by passing the PA).
HB