Why use drilled or slotted discs?

Drilling or slotting discs aids the disc in several ways:

The edges of the slots or holes continuously clean and refresh the pad surface as well as providing increased brake "bite". Additionally, they prevent gasses from collecting between the pad and disc interface.

The disc is lightened, thereby decreasing its rotational inertia.

Improved ventilation increases the disc's ability to shed heat, resulting in cooler operating temperatures