Why is it important for a teacher to understand human growth and development?
I can think of two reasons why it might be important for teachers to study and understand human growth and development. First, if a teacher intends to teach a course on such a subject, it would be wise for him or her to have a thorough understanding of human growth and development before teaching others. Secondly (and more importantly, in my opinion), knowledge of growth and development is vital to creating appropriate lessons. Knowledge of physical, mental, and social development in humans is important for teachers to consider when developing their lesson plans.
Let's consider the importance of understanding physical growth and development. Teachers either should not have students perform exercises they physically cannot do, or may choose to alter the exercise so it is appropriate for the student. Children and adolescents are in a period of development where they are still "getting the hang of" their bodies. The development of fine motor skills is especially important for academic tasks like writing, typing, and crafting. Activities should be appropriate to the student's skill level. It would also be prudent for a teacher to have an understanding of human growth and development and problems which may arise in exceptional circumstances. If a child suffers from pain in their legs during or after a physical education course, teachers should have some understanding of the possibilities of fractures or nutritional deficiencies.
Mental development is similarly appropriate to creating lessons which meet and challenge a child's skill level. Children may be in varying stages of development which hinder or allow them to practice abstract thought and object or personal permanence. Abstract thought is especially important to subjects like math and science, which may involve lessons about things which are intangible. Children typically do not study physics both because they do not have foundational knowledge of the subject and because their brains may not be entirely capable of the functions necessary to understand the laws of physics.
Of the three of these, I think social development needs the most additional attention in educational settings. Young people may be maturing at different rates or have different experiences with socialization, and teachers should have a good understanding of social development and how it affects education. For example, if one student has been lacking in their socialization and experiences anxiety at school, the discomfort they feel can impede his or her ability to learn.