Dentists needs them to become a doctor of dentistry. At least 1000 for a scholarship, 10,000 for a doctorate licence. Also they need a tooth to DNA test you so you are no longer anonymous online or in the real world.
Actually is it parents who save baby teeth in photo albums. To link to the children when they are older.
They are not just collecting the teeth of young children, they are collecting part of their soul innocence-essence that disproportionately reside in their baby teeth and is discarded as the child ages. The fairies use this essence to craft a barrier specific to each child to keep trolls from stealing it. Once all of their baby teeth are gone they are no longer tied to the old world of magic and trolls are locked out of stealing any of their essence. Trolls that succeed in stealing essence can exact influence over the individual even into adulthood by limiting their access to it. These adults become bitter and cold. Fairies refine and reintroduce the captured child-essence into adults throughout their lives. Adults who are given their innocence-essence back continue to learn and bask in the wonder of life just as they did when they were children.