[Piotrsko]

All these years I have believed the main purpose of VG's were to keep the airflow attached, typically at sub critical Reynolds numbers because when your flow becomes detached you fall out of the sky stalled, at any speed. I also understand that at some distance from the surface, the airflow returns to being laminar so I suspect dimples or divots are just keeping the surface from stalling (detached airflow) which for wings in flight is the highest drag point.