I moved to Germany in my late 20’s and lived there for 2 years. I went with perfect teeth; I returned with several cavities. My brushing habits did not change, and while you could blame my cavities on several factors (the food was different; I drank way more beer; I got two years older), I've always attributed it to the fact that Germany does not fluoridate their municipal water - and everyone drinks bottled water anyway. At least, the crowd I ran with did. It's been years, but as I remember, you didn't get water at restaurants unless you asked, and then if you did, you got bottled water. In any case, I went from drinking fluoridated water regularly to zero flouride except what was in the toothpaste. And, honestly, I don't remember paying much attention to which toothpaste I bought, and many don't have flouride in them.

So, yeah. It's just one anecdote, and there could have been many other factors, but it convinced me about the importance of flouride. Now we drink municipal water (US), but it's going through a whole-house triple filtration system and I don't know how much flouride we're getting. So in addition to the usual tooth care (brushing, flossing) I also rinse with a flouride mouth rinse, and I've been doing OK dentally.