Fluoride began to be added when it was observed that that kids living in communities with higher amounts of fluoride occurring in the drinking water had far fewer cavities than kids in communities with lower concentrations of fluoride.

Dentists and public health officials began to educate politicians and citizenry about this, and advocate for adding fluoride to the drinking water.

Imagine, dentists trying to accomplish something whose result would cut into their income: fewer teeth to drill and fill, or pull.

They succeeded in many areas, with a huge drop in the incidence of dental caries resulting, in those areas where fluoride has been added to the public drinking water.

At those concentrations, there is no reputable evidence that it causes cancer or any other undesirable outcomes, with the exception of rare dental fluorosis, which, again, is due to inadvertently getting too much fluoride.

The reason to put it in the drinking water is to get the fluoride incorporated into the baby teeth, but more importantly, into the permanent teeth, while they are developing. Applying fluoride to the outside of the teeth at the dentist's office, or with tooth-paste, seems to be less effective than if the fluoride is distributed through-out the tooth. Both processes seem better than either by itself, however.

As has been mentioned, a quick definition of a poison is "too much."

Too much of any good and necessary thing will shorten your life. Water is just one. Breathing 100% oxygen, which is absolutely required at the 21% concentration in our atmosphere, does bad things eventually to your eyes for example. 100% O2 for too long is too much of an otherwise good and necessary thing.
Ditto for fluoride. A very little is great; too much ranges from bad to very bad, again, depending on how much.