Brushing your teeth can reduce your chances of contracting a periodontal disease, as it reduces the bacterial build up in your mouth. Periodontal disease can increase the risk of developing cardiovascular disease, arthritis, even pneumonia. Rare cases but encountered.
Generally speaking if you don't brush your teeth regularly, bacteria can build up to form a clear sticky film on your teeth called plaque. Brushing your teeth regularly helps to remove plaque and prevent it from forming. It's important to avoid plaque, since it can cause tooth decay (cavities) and a gum disease known as gingivitis.