Answer:
Topical fluoride treatment in a dental office is a preventive measure. Fluoride applied to the surface of teeth strengthens the enamel and makes the teeth more resistant to decay.
Is this prevention 'necessary?' It depends on the individual. If the person has a history of dental cavities, then yes, use of fluoride would seem prudent and reasonable to prevent or reduce future cavities. On the other hand, if the person has reached adulthood without ever having a cavity, fluoride treatments may have little or no benefit.
Another consideration is the person's current health status. If a person is being treated with radiation to treat cancer in the head or neck region, there is a high incidence of increased dental decay because the radiation tends to cause the salivary glands to reduce their output, leading to 'dry mouth'. This lack of salivary flow contributes to dental cavities. Use of a topical fluoride can minimize this effect to some degree.