Because it prevents cavities. All children and adults (with teeth) need to have their teeth protected with fluoride. It hardens the enamel and root surfaces and makes them more resistant to decay-causing acids. Adults who have fillings and crowns have invested a lot of time and money in restoring their teeth. Fluoride helps protect this investment, similar to car wax for your car. Every day we drill out old fillings and old crowns that have decayed under the edges and need to be replaced. Fluoride can help prevent such repeat repairs. We get fluoride treatments for ourselves at our dental cleanings and we use prescription-strength fluoride at home because we believe it works. No cavities in 35+ years is proof of this.
Because they work where fluoride doesn’t, in the deep pits and grooves on the biting surfaces of back teeth. These pits and grooves collect bacterial plaque because they are too narrow and deep for even the thinnest toothbrush bristle to clean out. Sealants bond to the enamel and fill and seal these spaces, like floor wax protecting your floor. Cavities can’t form on the biting surfaces of teeth that are sealed like this. Sealants, together with fluoride for the smooth sides of the teeth, can help you be cavity-free for years. Prevention is always cheaper than repair.