It isn't, and the people who buy them are not, for the most part, race car drivers either.
I mean no disrespect towards the people on this site, but the
vast majority buy slotted and drilled rotors because they look cool, not because they've been proven to actually work better on the car.
Do they work 'better?' The best any of us could honestly say is
maybe. As far as I know, nobody's actually tested how slots and/or holes affect stopping distance, brake temperature and longevity.
I intend to buy new brakes for my SVX very shortly. I will either have them slotted or leave them solid. Honestly, I'm waiting on my own unscientific study to decide. My friend and I put slotted rotors on his car, which we will both be autocrossing this year, and I'm waiting to see how that affects the car.