As a teacher I can understand this. It's less work for the teacher if they don't have to constantly grade homework, participation, etc. It also makes sure the students understand the concepts they've studied and can use them.
Of course there are problems as well. Making a test that's reliable and valid is difficult. And you also have to think about objectivity and subjectivity. Not to mention that some students simply don't test well.
How heavily a class is based on exams depends on a lot of factors, such as the class, the teacher, and the admin. It's hard to find a balance that pleases everyone.
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