what is the benefit of additional shader cores
The more shader cores, the faster the shading can be applied during rendering.
and VRAM?
VRAM is much faster than shared system RAM when it comes to memory usage for the video. A video adapter having it's own VRAM relieve the system RAM of the video tasks, so that the RAM can be used to make other (ie: non-video) functions perform faster.
The more VRAM on the card, the less system RAM will be used for video functions. Of lesser importance in today's world of multi-GB video cards, the amount of available VRAM generally defines how high of a resolution (and colour depth) the card can support.
What kind of applications require the maximum number of shader cores and 8GB of video RAM?
Graphically intensive ones that are generating/rendering images on the fly. Games, CAD, etc.