Why a teacher needs to study child development?

Because understanding developmental stages and developmental tasks helps educators know what is reasonable to expect from learners. Students come with needs for growth that is cognitive but they also come with needs (and trends) that are emotional and behavioral. In other Words, according to developmental stage (like Early Childhood, or Early Adolescence) learners have to process and master tasks that are both academic and social-emotional. If educators are not cognizant of this (or its importance) it is hard for teacher and learner relationships to be mutually respectful on an interpersonal basis. Without such mutuality, teaching and learning are complicated with all sorts of obstacles.