Why is sodium fluoride put in our water?

It reduces cavities. When Fluoride is absorbed into the body of a child developing teeth, the Fluoride gets incorporated into the calcified matrix of dental enamel. This Fluoride reinforced enamel is more resistant to tooth decay than enamel without fluoride. This reduces the incidence of cavities and promotes healthier teeth.
Decay. To prevent decay and it has worked fantastically well especially for children.
Small amounts good. Like many things (vitamin c, iron, oxygen, water), Fluoride in the correct small quantity is safe. It is only toxic when systemic exposure is much higher then the recommended amount. At the correct amount Fluoride makes your tooth enamel more acid resistant and lowers tooth decay. That is why small amounts are in drinking water. In reality you need a lot over a long time to have any problems.
Prevents decay. Water fluoridation has been one of the greatest advances in health care in the 20th century. It has been proven time and time again that Fluoride makes enamel stronger and reduces tooth decay and has no detrimental side effects. It aids in remineralization of enamel and also has antibacterial properties. Fluoride occurs naturally in ground water in some areas.