It, quite effectively, fights tooth decay. It's a dirt cheap option in places that have lackluster oral hygiene to raise overall population health. It's marginal in places (one would assume like the first world.) but it also has no draw backs. It doesn't hurt, doesn't do anything long term if you already have decent hygiene, and it's (again.) dirt cheap. Doesn't replace brushing your teeth, but it helps overall and long term physical and oral health.
And I know all this because montreal is once again fighting to stop adding it to the local water supply. The city with the most lead in the pipes in north america is fighting to get the flouride out of the water.