Graphics cards are used for creating the actual display on the screen that you can see. They go beyond on board graphics on Intel systems, and most computers nowadays have graphics cards in them anyway. Many people buy their own high-end graphics cards to place in their system to allow them to play games at max settings or use editing or 3d rendering software on their computers which can be some graphics demanding applications. Some people purchase lower end graphics cards or already have low end graphics cards in a prebuilt system to accompany a motherboard without onboard graphics or to supplement the graphics already in the system.