This is used to fill in narrow grooves in a tooth that cannot be adequately cleaned by brushing. In some cases, the tooth structure has fine grooves or pits which accumulate plaque, not because the person doesn't brush, but because they're too narrow to allow even one bristle into them. These will develop cavities over time, and you don't want that. The dentist will brush on a coating that seals the grooves and pits, making it possible to brush off all the plaque and keep your teeth healthy.