The funny thing is that I really don't give 2 damns about graphics cards so I'm just gonna put the third one lol. Why would you spend $700 on one anyway?
Graphics cards let you render graphics faster.
Also, due to the insane amount of cores (3.5k in this one, compared to 4 on an i7 6700k) it is extremely useful for getting mathematical results fast.
It also has more VRAM, so you can hold more textures and other data.
And spend 700+? because, fast amazing graphics. Example, let's say a computer gets only 70 fps at min settings without a GPU, with a gpu like this it might get 150FPS at max settings (depends on game and cpu ofc, cpu might bottleneck it or the cpu might be really good and not as much of improvement but still a lot of improvement).
tl;dr: it's another cpu with maor powwerrrrr