The chewing of sugarless gum increases the flow of saliva, which washes away food and other debris, neutralizes acids produced by bacteria in the mouth and provides disease-fighting substances throughout the mouth. Increased saliva flow also carries with it more calcium and phosphates to help strengthen tooth enamel.
The only varieties of gum with the ADA seal are sugarless. Don't let chewing gum replace brushing and flossing. It's not a substitute.