What is the purpose of length requirements in the writing classes we took in high school and college?

Perhaps the argument behind requiring a student to write a 10 page essay is that it ensures that the student is writing enough for the teacher to grade. By requiring 10 pages, it allows the student to scope out their idea in as many different directions as is necessary for the teacher to grasp what they are trying to argue.

But I would argue that it rarely accomplishes this goal. Rather then needing the 10 pages to elaborate a well-thought out idea, students will write just in order to fulfill the page/word requirement. Writing verbosely until they have enough. So what does this actually teach students?

It teaches us to communicate ambiguity and long-windedness in a culture where neither of those are important or desired. It teaches us that ideas do not need to be refined or edited but rather we can submit our stream of consciousness-thought.

Perhaps instead it would be useful to have the student first write the 10 pages in order to find their idea, and then edit it down to 5 pages to make it clear and concise.