Comment by Hayarotle

Essays are often meant not as a measure of understanding, but as a tool for the student to learn, to reflect on how well they understand the material, and for the teacher to be aware of their weak points. Sometimes they're also meant as a tool for training the skill of essay writing itself. In those cases, it's only graded in order to nudge the student into reading the material and studying (and sometimes to cushion the impact of test scores).