Why are disk brakes not 100% round? Why is the outside edge "rough"?
For the brakes to self clean and remain effective, the surface of the pad must transition from metal to void at least once every revolution across the entire pad (i.e. no continuous line of metal). The 'sawtooth' rim achieves this. Some very cheap disks do not achieve this.
You may find the very good answer by @trailmax in this question useful.
I'm relatively confident in saying that bike disk brakes have such outlines because it makes them look "cool" or "fast".
The great, great majority of car disk brakes have no holes, and they work fine. "Performance" car disk brakes have holes and/or slots, but they are absolutely not required.
The general reasons for holes or slots in disk rotors are:
- Improvement of cooling, by increasing surface area and breaking up laminar airflow, as well as (for car rotors) adding an additional airflow path.
- It can serve to reduce the rotating weight of the brake disk.
- In wet or dirty conditions, it helps water or dirt escape from the brake-pad brake-disk interface, which improves braking performance in such conditions.
As such, while holes or other shapes cut out of the actual brake surface are reasonable, I can't think of any good reason for the non-circular outline, except perhaps some minor improvement in cooling due to it acting like a blower fan a little tiny bit.
Note that this is not a vented rotor, but rather a cross-drilled rotor. Car disk brakes do have internal ventilation, but it runs parallel to the brake surface, rather then through it.