Cross drilling the disks helps with heat loss, as the friction generates an monuments amount of heat through braking. Also for water displacement, If i'm not mistaken, it was first used to displace the gas/dust the old asbestos pads generated.
Another purpose is to reduce unsprung weight of the suspension which improves handling.And yes cooling improves braking.When brakes get very hot they fade.So the cooler you keep things the better(and longer) the brakes will function.
it's helps cool the rotors, same with slots.
