It's my understanding that the primary reason for origination/invention of cross-drilling rotors was to reduce heat. When the brake pad grabs the rotor, it creates friction, which creates heat. If that heat can't escape, it leads to brake fade, which reduces the brakes' stopping power. A secondary reason was to reduce gas build up between the brake pad and brake rotor. The materials used in some older types of brake pads caused gas to build up between the rotors and pads. That gas also limited stopping power. A third reason was to displace water between the brake pad and rotor. I believe that reducing weight was a beneficial by-product.