The reason is to dissipate heat under hard braking.
The original intent for drilled rotors WAY back, was to dissapate gases that would build up between the pad and the rotor during hard braking in order to avoid pad fade caused by the trapped gas. Pad formulations have changed significantly since then, and eliminated this gas buildup as an issue, even in racing pads.
Your statement, taken generically, is wrong. There are specific instances where it is true, but in general, it just doesn't work that way. There are some OEM and racing applications where the entire rotor is designed with the intent to have the holes drilled in the faces, and the airflow is modeled in such a way to take advantage of it. It takes a significant amount of work to make it effective. Even then, its only used in cases where it is necessary to have more rapid heat dissapation at the expense of rotor life. Applications such as racing and very high end sports cars.
When you consider passenger cars, using rotors not originally designed to take advantage of the drilled holes, you can actually REDUCE heat dissapation by removing the pressure differential between the inboard and outboard edges of the rotor vents, and drastically cutting bulk airflow through the rotor.
Even most of the OEM drilled rotors you see are still just done for appearance reasons, this includes OEM applications from Porsche, Ferrari, etc etc.
Slotted rotors are a different bag altogether, and have nothing to do with cooling. Slotted rotors were developed for racing applications in order to "scrub" the face of the pad in order to ensure a constantly refreshed pad surface. In cases where the pads can get "clogged" with scorched pad material, or even overly wet, this can improve initial brake bite, and sustained heavy braking can be a little more consistant. This is all at the expense of higher pad wear.
All that said, no rotors exist for Contours that were developed from the ground up to utilize drilled rotor faces. All of the drilled rotors you will encounter are done PURELY for appearance, despite whatever marketing claims you may hear. Even if they improved heat dissapation by 20% (the best data I have seen says no more than 10% is common) you wouldn't even have a chance at noticing it on a street driven car.
Even slotted rotors provide no viable benefit on a street driven car unless you have selected a very poor pad for your car. Street tires are the limiting factor in those cases, not the more aggressive initial bite possible with slotted rotors.
In conclusion, stop regurgitating old wives' tales, especially when you don't have experience in the area.
