For most people, the fluoride treatment isn't their favourite part of a check-up. However it's been well established that fluoride is beneficial for teeth.
Not only does it strengthen teeth to protect against decay starting, but it can re-harden early decay and even has a mild antibacterial action.
However if you ever wanted to look at an example of the difference it can make, look no further than country NSW. Two towns. Only 48km apart. Yet light-years apart in tooth decay rates.