Germs that cause gum disease, gingivitis, and tooth decay live in groups. These bacteria are much less destructive when their groups, called colonies, are mechanically broken up with a toothbrush. Another problem caused by bacterial colonization is the production of foul odors. This is why proper oral hygiene is important.
The surface of the tongue is covered with many little tissue projections, called papillae, which serve various functions such as detecting taste. Papillae also provide protection for bacterial colonies. Since foul odors originate from bacteria, an unclean tongue is a major source of bad breath, or halitosis.
Brushing the tongue is an effective way to manage halitosis. It also reduces the overall amount of germs in the mouth to help prevent gum disease and tooth decay. Some dentists also advocate scraping the tongue routinely with special tools as treatment for halitosis. Probably the most effective time for brushing and/or scraping the tongue for the management of halitosis is when brushing the teeth in the morning.
If you are brushing your tongue, but still suffering from bad breath, you can see additional causes here.