"Light is the language of Photography," Joe McNally says. Artificial light comes in all colors and can really throw your camera for a loop if you're not aware of it--it'll be like a foreign language and you won't understand why your picture doesn't look like you imagine it. White Balance can be adjusted according to the type of light you're shooting in (sunlight, fluorescent light, normal light bulbs, etc.). The default setting is automatic, which usually does a good job of making the light look normal--it's like a gramaticaly correct sentence. But sometimes, you don't want "correct"; sometimes you want to use "colorful metaphors".
The auto white balance made the neon lights of the first picture a much more pale green and also washed out the blue and the red on the sides of the face. I adjusted the white balance to fluorescent--specifically for mercury vapor fluorescent. Now we're singing with light.
Similarly, this image, shot under a street lamp, was a very boring and flat orange color on auto. It was like the weather report--it's something everyone who goes outside at night can see any day. I changed it up and used the mercury vapor setting giving this magenta tint that lends a different mood to the image. Now instead of recciting the percent chance of rain, we're ready for poetry.