If you're a kid and building teeth, every bit helps. For adults it doesn't do much.

The movement to add fluoride to the water of places that don't have it naturally started because the kids in the places that naturally had fluoride had better teeth.

It's about the cheapest most value positive public works project you can think of. Add a mineral at the place you're cleaning water anyway, improve the life of every child downstream, and it would take orders of magnitude higher concentrations to have even slight negatives for anyone involved.