Incisor teeth play a crucial role in maintaining proper vertical dimension and occlusal relationships. Extracting incisors can disrupt the anterior guidance, leading to a loss of vertical support and causing a tendency for the bite to deepen abnormally. This results from the over-eruption of opposing teeth and increased mandibular closure, which can compromise both function and facial aesthetics. Therefore, preserving incisors is important to prevent the development of an abnormally deep bite and maintain occlusal stability.

Reference: Essentials of Orthodontics, Richard Lee, 4th Edition.