Most people use toothpaste when they clean their teeth (you'd be surprised how many don't......).
Most people are aware that there's usually some fluoride in there, along with little cleaning granules and some minty flavour.
Most toothpastes even have something to kill mouth bugs and some have stuff that stops sensitivity.
Some even claim to whiten your teeth - which they don't, so save your cash.
But none of the above works if it doesn't have time to soak in to your teeth. So, it follows that the toothpaste must remain in contact with your teeth long enough to have the desired effect. Even if you do brush for the recommended two lots of two minutes twice daily, you won't get the benefit if you then take a big old mouthful of water and rinse it all down the sink - think about it!
So - spit the excess toothpaste out at the end of brushing, but don't rinse! Better still, make sure your toothbrush is the last thing that goes through your mouth before you hit the sack.
Does that make sense?