Brake rotors

Due to advances is pad materials they are not really required or better than solid blanks. In fact they can crack where the holes are. One could argue slotted rotors help clean pads.

The purpose of them originally was older pads would out gas, causing what they called "Green Braking". The gas build up diminished braking, so ports we're put to to allow the gas to escape. Slots were a away to to prevent cracking.