I use lightroom and there is a little dropper that you can place over into your image and it locks into the correct white balance pretty consistently. But you need the right subject to take advantage of this. Someone here said it and it works amazingly. A white styrofoam cup. Bam. Run the dropper over that and perfect. Then copy the setting to the other images that were taken under that same light.
As for the purpose of white balance... if you have the wrong setting, it will look too blue, too green, too red, or too yellow. All of which can ruin a photo.
I have been shooting in Raw the past 6 months. So someone correct me if I'm wrong, but white balance only affects the jpg image... I think. So if you are shooting in raw, it won't matter.