In Miss Peregrine's Home for Peculiar Children, the main villain Mr. Barron (played by the ever talented Samuel L. Jackson) is a shape shifting Peculiar, turned Hollow and wight. But one thing he seems to have unlike the other wight is very gruesome teeth after his transformation back to humanish form. Why is that? It's not like he couldn't shape shift them back to normal human teeth since he could shape shift completely.
When Barron turned into a hollow, he had to eat several eyes to get back to looking human. For a wight to look fully human, they must consume as many peculiar eyes as possible. Except, they will always have their distinguishable white eyes. None of Barron's friends had transformed completely. One of them had a rat like appearance, and Barron still had hollow teeth. He said later during the movie that he had "searched the globe in hope for one more peculiar eye.", which would transform him completely (except for his eyes).
Mr.Barron was a Hollow and he needed to eat other peculiar's eyes to get his humanish form back. Maybe he didn't eat enough eyes to transform his teeth, or there were some parts of his body that couldn't be transformed, like his eyes and teeth.
Or maybe he liked his teeth that way (It makes him scarier).
For the utility. Sharp teeth are scarier and more dangerous.
We know that he can transform his teeth, which he did several times:
As the bus driver
As the psychiatrist
As the gardener.
There may have been some other times that he didn't mention.
Transformed his arms and turned them into weird bladed weapons.