What is the importance of developmental psychology to a teacher?

A teacher will be expected to deal with students in their early years, when they are beginning to develop who they are and what they stand for. In youth, people can often have very different ways of thinking from mature adults, and teachers need to be prepared for this. 
Most colleges have teachers take classes in psychology or developmental psychology with the hope that teachers can apply what they learn to teaching students who are still developing as people.
Instances in which this training can help teachers include understanding the lack of logic and reasoning behind young students, defending against the pack mentality of youth, and maintaining authority over children. Developmental psychology is very important to teachers and educators of all types.