Assuming that the prescribed AEO rotor speed for a given helicopter model is 100%, some manufacturers have determined that (at Vtoss and Vy) better OEI climb performance is achieved at a slightly reduced Nr (say 95%). This likely occurs because the rotor power required is reduced, while the engine continues to produce virtually constant horsepower within about +/- 5% of its design operating RPM.
For an OEI continued takeoff following an engine failure, I believe the “most significant benefit” of drooping the rotor below the reference NR (100%) is that it assures that the engine is producing the maximum available power at the associated OEI limit (TGT, NG, Q), because the fuel control is working to increase the NR back to the 100% reference value. This is also a pilot workload reducer since the pilot only has to monitor Nr to be assured that the engine is producing limit power.