The reason for this is that we need to understand the ways in which children's attitudes and abilities develop so that we may have a better chance of understanding the children and how to teach them.
As an example of this, it can be important for a teacher of history and social sciences to know what sorts of things (like fairness) are important to children at any given time in their lives. If you teach students who are at an age where fairness is a paramount concern, you can interest them in your subject matter by highlighting issues of fairness in (for example) different historical periods or in different types of societies.
By understanding child development, we are better able to teach and manage our students.
Children do not all develop at the same rate, but they do tend to follow stages. A teacher needs to understand what a child at a certain age should developmentally be able to do. A big mistake teachers often make is to try to teach students something they are not developmentally ready for. The biggest example of this is algebra in 7th or 8th grade. Many adolescents are not really capable of the abstract thinking that algebra requires, so they won't do well no matter what.
To teach effectively child development is a necessity, not an optional extra. Of course, whilst every child does develop at their own pace, there are general guidelines and stages that we can use to inform our knowledge, that come in great use in terms of teachers knowing what students are ready for and also to provide some kind of basis which can be used to measure the progress of children.
Education is, in and of itself, a form of child development, and an important one, so for a teacher to be well versed in the study of child development is key to their effectiveness. This knowledge, if for no other reason, helps a teacher to appropriately plan and develop content and complexity of the lessons and curriculum he/she is going to bring to a classroom. There are physical, mental and social differences, stark ones, actually, between a primary 3 and a primary 5, not to mention differences in general ability and vocabulary, which is still very much in development at the primary level.
There are many scenarios that will come up in the classroom where it will certainly be beneficial for a teacher to have a basic understanding of child development. Children are not "little adults." Their emotional, physical, mental, and intellectual foundations are still forming. The disciplinary apporach that a teacher takes for kindergartners should differ from that of 5th graders. I can say that from experience my assertive discipline approach and having students take ownership and reflect on their behavior doesn't work well for the younger students as it does for the older ones. I prefer to work with older students.
And there are definitely intellectual stages of development as well. As someone mentioned, abstract thinking and other concepts take time to develop. Now it would be great if 'the powers that be' would realize the importance of developmentally appropriate curriculum and not have teachers teaching material that is too advanced for the age group to grasp.