Content knowledge is important in the field of education for two main reasons.
First, content knowledge is important because it is one of the main things that teachers are trying to impart to their students. We assume that students who pass a certain grade level will know certain things. We expect that they will have appropriate levels of math and science knowledge. We expect that they will know certain sets of words. We expect that they will know certain things in terms of history and social studies. All of this is content knowledge. Therefore, a major part of what education is about is imparting content knowledge.
Second, content knowledge is important because there is some degree of disagreement about its relative importance. The educational system is not all about teaching knowledge. It is also about teaching skills that transcend any particular subject area. It is about teaching critical thinking. It is about encouraging creativity. These things are not the same as content knowledge. There is disagreement about how much we should require the learning of content knowledge and how much we should focus on giving students the more broadly defined skills that they will need to function.
For these two reasons, content knowledge is both important and controversial in the field of education.