Thermal mass is essential in making the brakes work well. You're just changing kinetic energy into heat. The heavier a rotor the better it can handle more heat and harder braking.
When a brake is applied, all of the kinetic energy of the vehicle is converted into thermal energy.
Heat factors:
E = 1/2 MV²
E = Kinetic energy generated
M = Vehicle weight
V = Speed of the vehicle
Since all the converted heat needs to be absorbed and dissipated, the rotor comes into play as the “heat sink,”. As the rotor heats up, it absorbs heat just like a water sink holds water from a faucet. If the water pours into the sink at a rate faster than drain can handle, the water will overflow. Likewise, if the temperature of the rotor increases at a rate faster than the rotor can cool down, consequent damages are likely to occur. In one case, you end up with a wet floor, and in the other, a damaged brake system. In some extreme cases with too much heat, the tires could be set on fire.
Now we know how important a brake rotor is to a brake system. We need a bigger heat “sink” and efficient heat "draining" system to prevent heat from overflowing.
Increasing the holding (thermal) capacity means enlarging “sink” size, but in brake rotor instance this may be impractical, as it would also increase the rotating mass, which slows down the acceleration and as deceleration. This is not a desirable solution. So the challenge is to produce a rotor with the same mass, yet can manage or hold the same amount of heat longer, without overflow. To achieve this goal there are two main issues to overcome:
1. A better material that is more resistant to higher temperature with good thermal stability
2. Improved designs that can dissipate the heat faster