I guess that the holes do help with heat dissipation, but I've always assumed that they were there to provide somewhere for the water to go when the brakes were applied and the pads squeezed the water off the surface. I had a bike (Yamaha XS650E) which had twin front discs with no holes and stopping in the wet was an issue - especially since I had the extra weight of a sidecar. There was a noticeable delay between applying the brakes and getting any stopping effect. It felt as though the brakes had failed, and the more urgent the situation, the bigger the delay seemed to be! I developed the habit of keeping the brakes dry by lightly dragging the front brake for a second or so when the things going on around me were likely to present 'surprises'.
It was a revelation when 20 years later on my first ST1100 at how instantly available the braking was in the wet - but even now, just occasionally, I still catch myself drying the front brake if traffic ahead is getting interesting !