They add it because it makes people docile and cooperative. Studies were done by Germany before and during WWII to identify chemicals that would make the people of invaded countries less hostile. Fluoride compounds were idenfied in this study.

Fluoride was introduced into water in US to hide the fact the hydrogen fluroide was used to enrich uranium in the Manhatten project n*ke weapon secret project. This resulted in fluoride levels increasing in the areas around the purification plants. So they introduced fluoride into the water to act as an explanation for these increased levels. The whole spin on fluride being good for teeth was created as a disinfo psyops.