It serves to effectively lengthen the chord of the wing (flap section in this case) by delaying the point at which the upper and lower air streams 'rejoin'. I assume on the van it is there for when the flaps are down, and not for the cruise condition. I can only assume it is there to slightly lower stall speed. It is probably something to do with the fact that singles have to have a certified stall speed in the landing configuration of 67kts or less (I think, from memory). As the 208B was a stretch/IGW version of the original 208, the 208B probably needed to generate that little bit more lift for certification. The flap leading edge VGs are there to re-energise the boundary layer to prevent/delay separation.