The 105 % is nice in case of the engine failure, where that energy can be extracted as the rotor droops just after the failure. For the few seconds, it can be like having 10 to 20% more power on the remaining engine, a nice power boost!
If the engine does not quit, and the rotor does not droop, the extra rpm is not very useful, and might actually hurt climb performance a bit, as the rotor might be less efficient at higher rpm (for the typical helicopter.)
Cat A's are the most performance critical takeoff, and the only one we spend a lot of time on, so the rpm boost is usually specified only for the Cat A.