Summarize the following:
The tongue's unique texture and large surface area makes it host to millions of bacteria, which can travel to the teeth and gums and cause cavities and gingivitis.  The teeth gather plenty of bacteria themselves, but unlike the teeth, the tongue is not a smooth surface. It follows that even more bacteria accumulate in the taste-buds and small pits on the tongue.  Just rinsing the mouth with water is insufficient to remove the bacteria on the tongue, primarily because the bacteria gather in what is called a "biofilm"-- a sticky, gooey, living layer of bacteria. It must be disrupted by rubbing or scraping to fully remove it. The bacteria that live on the tongue cause a chemical reaction that produces volatile sulfur compounds, which results in bad breath.  Tongue brushing can also remove traces of odorous foods like garlic and onions, which can also improve your breath. Many people with bad breath are not aware that they have it. Its a good idea to practice good oral hygiene, including regularly cleaning the tongue, in order to reduce the chance that you are one of them. Certain foods and health conditions can cause a colored or filmy appearance on the surface of the tongue. Brushing the tongue can eliminate this problem.  Foods containing dyes like cupcakes, lollipops, or popsicles can temporarily dye the surface of the tongue. Brushing the tongue can reduce the coloration of the tongue, making it less noticeable to others. Some illnesses like oral thrush or syphilis can cause a buildup of bacteria or fungi on the tongue that cause a filmy, white appearance. In some cases, the white film cannot be brushed off. Be sure that you see your doctor if you suspect that your filmy tongue is caused by an illness.  Certain medications like antibiotics or Pepto-Bismol as well as certain illnesses can also cause a startling discoloration of the tongue called "black hairy tongue," which is caused by an overgrowth of yeast in the mouth. The condition is not serious and the discoloration can be brushed off with a toothbrush or washcloth, although it might be a good idea to see your doctor to determine the cause.

Summary:
Prevent bacteria build-up. Prevent bad breath. Prevent discoloration.