In vowel formation, the teeth are almost the last things the vowelled tone comes into contact with, before launching towards the listener’s ears. Since they have a hard surface, they are reflective, with the shaped air molecules bouncing off them.
The vowelled tone should not be directed to the teeth in any conscious manner, but the teeth play an important role in adding final touches. Just think, if we had no teeth, there would still be a recognizable vowel, but the vowelled tone would be different, as if it were “fuzzy” or a bit “undefined.”