multiple gpu increase performance in games like sli and cfx. it also increase performance in work related programs too.
both game amd work programs need to support it, in order to take advantage of it. if not then it's just a waste.
multiple gpu is also use for multiple monitors. these days a single video card, most cnm display up to 3 monitors. Some may need more than that either for a video wall, to watch their stock charts or work that benefits when using multiple displays.