There are some people that run multiple monitors at a high resolution. When the resolution is extended across multiple screen, that creates more processing power for the Vid card. So, in order to, keep the FPS at a reasonable rate on such a high resolution, multiple cards are used.
Some people just like having two Vid cards in their system, because they think it will produce more horsepower for their gaming. Problem is, most people use single monitors, and chances are, they are barley using the second vid card.