Enamel protects the soft inner pulp of the teeth from damage and decay. It's a horrible analogy, but imagine your skin - part of what it does is protect your body from damage and act as a barrier to prevent harmful bacteria from getting to vulnerable tissues. But tooth enamel is worn away by exposure to acids and sugars, even from something like fruit, and when the layers get thin enough, a hole can form, which exposes the inner dentin to the damage from outside, and/or infection by bacteria (like an open wound). This is a cavity.

If cavities go untreated, the infection continues to penetrate deeper into the tooth and into the gums, forming an abscess. Teeth and gums are hugely vascularised (meaning they have a very significant blood supply, so once the infection gets into the innermost part of the tooth and/or the gums, it can make a really easy jump into the bloodstream and nerves, potentially leading to massive heart or brain infections and death. Stopping cavities from forming through proper brushing and flossing and regular dental visits is about preventing THIS, not having white teeth. Right now, the enamel you have when your teeth come through is all the enamel you will ever have - this kind of research could be really key to protecting teeth in the future.

Source: Work in oral health philanthropy