Traditional story was the tooth fairy takes your tooth in order to put it in the sky as a star. That's why we brush, to keep them nice and white. That's why a coin was the reward, to look like the moon.
ps -- My son graduated from silver dollars to $2 bills thanks to some smartass parent of a kid in his class at school. Upward peer pressure. lol
On rise of the guardians movie she tells them that that's where all of your best childhood memories are kept so she takes your teeth to keep those memories safe
Lol , the tooth fairy takes your teeth to take care of them. And if you brush and take care of them she will leave you something special.