The reason you need to do this is because, regardless of what your eyes tell you, the color of an object is extremely dependent on the quality of the light source.

This is quite obvious when using camcorders or color slide film in photography. If you take a picture of an object indoors under incandescent lighting, and then move the object outside on a cloudy day, and take another picture, and compare the two images without adjusting for white balance, the color of the item will appear very different in the two locations. In digital imaging, one sets the white balance by adjusting the baseline. In color slide photography, you either use film calibrated for your light source, or you use filters to compensate to the difference. The reason you dont' have to think about white balance with normal color print film is because they do the color balancing at the time of printing the photograph. Indeed, if correct color is important to you, you take a picture of a chart with calibrated colors, and you specify to the printers to adjust the settings so that the colors on the chart are correct.