I helped instigate this in the S-76A back in the early days, and we still use it. The OEI climb rate is better at low Nr (the rotor is more efficient at lower speeds), also you extract maximum rotor energy during the critical low speed fly away and finally you can be sure the engine is producing maximum power if you pull against the limiter, so the procedure is written that way. The climb difference at 93% Nr is small, but worth it.
When you achieve Vy, you are able to transition to normal flight, and the lower Nr isn't needed. To prevent the need for a special Vne and chart at an oddball Nr, the procedure asks that you not go above Vy at a low Nr. If we permitted you to fly to a higher speed, we'd have to publish a Vne for it. It is more convenient for all of us to simply remember to get to 107 when above Vy.
In test, we flew the A to 150 knots at 96%Nr at weights up to 10300 lbs, so you will do no great harm to the rotor to slip up above Vy by a few knots.
