When energy causes a mass to spin, the distance most of the weight is from the center of the rotation is important. If the weight is close to the center, not a lot of energy will be needed to accelerate it or slow it down. If the weight is far from the center, more energy will be required to speed up or slow down the mass. When those two examples are spinning at the same speed, the one with the weight far from the center has more momentum. This is why a light baseball bat is easier to swing than a heavy one, but can't hit a ball as far; the heavy bat has more momentum, and that energy gets trnaferred to the ball.
In cars, rotational inertia is important during the engineering process. It can greatly affect braking, accelleration, and engine performance.
Rotational Inertia's Effects on Horsepower
Energy (in the form of torque) is required to increase the rotations per minute of an object, and any energy being used making that happen doesn't make it to the drive wheels.