that's really a terrible misconception. it works topically, in the sense that your body delivers it via saliva. but, to actually do anything, you need a sustained source.
so, if you brush your teeth with high potency fluoride, it really doesn't do what you think it does - you'd have to leave it on your teeth for ten, fifteen hours at a time, and of course you can't do that. but, if you get it in your saliva, you get constant slow release, which is actually effective.
when they varnish your teeth, you're supposed to leave it on for 12 hours or so - and that's 23% fluoride.
so, what does this .1% fluoride toothpaste do? the answer is "not much", and that people arguing that it's ok to take the fluoride out of the water because you can just brush your teeth, and it works topically, are badly misinformed.