What does a rotating mass actually do?
A rotating mass does not really consume or dissipate energy. A rotating mass stores energy. The rotating mass eventually either returns energy to the system in a useful way, or something converts the stored energy to some other form of unwanted energy. The conversion might be with a friction, converting to heat. The energy stored might be helpful, like the smoothing of cylinder pulses in an engine flywheel. The energy stored also might not do anything at all, or the stored energy can even be harmful, reducing acceleration or braking.
Accelerating an unnecessary rotating mass requires energy, and the acceleration process saps some of the horsepower we have available to accelerate our vehicles. Reducing available horsepower affects acceleration in a very predictable manner, and the horsepower amount needed to spin something up gives us some feel for how important a part change might be.