A computer in a computer
The thing about what it takes for your computer to process your game. The computer must receive input from the player, interpret that input, and then apply the results to the game. Once the input is completed, the computer must handle the physics of the game: objects must be moved, collisions must happen, and explosions must ensue. Once the computer has completed updating all of the objects in the game, it must then render these results to the screen. Finally, in order to be convincing, all of this must occur at least 30 times a second and often 60 times a second!
It is truly amazing that computers can process this much information that quickly. In fact, if it were truly up to the central processing unit of your computer to accomplish this, then it wouldn't be able to keep up.
The 3D graphics card solves this problem by taking care of the rendering process so that the main CPU of your computer doesn't have to. All your CPU has to do is deliver the data and the graphics...