White balance is typically used to make the colors in your pictures look accurate. The goal is to make white things look white and all the other colors look like they do in real life. However, white balance can also be used to change the mood of your pictures. Changing the white balance can make pictures seem warmer or cooler (some may say harsher.)
Most of your camera's white balance settings change how blue or orange your image appears. (The fluorescent setting does not fit in this category since fluorescent lights are greener than other light sources.) Because of this, intentionally using a WB setting that is not the most accurate can make your picture seem warmer or cooler than it actually was in real life. Consider the following pictures:
This shot was taken on a gloomy, overcast day. And it looks like it because the WB was set for cloudy.
This shot was taken a moment later with the WB set to shade.
Notice how much warmer this image looks. It may not be reality, but it's a nicer picture to look at. Try experimenting with your WB settings so you know what they do to your pictures. Then when you want to make a slight color adjustment to you pictures, you can use your WB to do it.
