This is a good question. Teacher assessments are very important for students for many reasons. Let me give you a few of them.
First, one of the primary objectives for teachers is not only to teach, but also to assess how students are doing. This assessment is very helpful for students, because students can know how they are doing and through the assessment learn how they can do better. During this time, students can also learn where their weaknesses and strengths are. This feedback is critical for all students and one of the key ways they can become better students.
Second, teacher assessments are important for teachers. They are able to gauge how the students are learning. In light of this, they are able to modify their teaching styles and content to help students learn.
Finally, if this is an elementary school or a high school, then these teacher assessment can be very important for parents. Parents should also get involved in the education of their children. Moreover, periodic assessments are very important to have, so that there are no surprises.