Those custom FET's haven't yet publically been torn down. But I'd guess they enable more efficient motor inverters. More efficiency means smaller battery, and less cooling. Less cooling means the whole heatpump system can be downsized, reducing weight. Reduced weight means smaller battery. Smaller battery means less weight... Which means an even smaller battery... And a smaller battery in a lighter car means more profit margins.

Because of the recursive nature of this, even small efficiency gains on the FET's have a pretty massive impact on profit margins.