In motor racing, often, duct pipes are built to direct the cold air towards the discs. Disc brakes in motor racing almost get too hot and need a lot of cold air to cool down before the next corner.
Motor bikes usually have only one plate, so they don't need to be ventilated as they have free air on both side of the plate. A car is a lot heavier and to stop it, two plates are needed. Using one thick plate instead of two thin ones would require a lot of cooling down. Cars also don't have as much air
circulating around the brakes as motorbikes do. So the best solution is to have air circulating between the two plates.