Surprisingly, teachers have been known to assign writing tasks without articulating to themselves what the task is supposed to do for students. Good writing assignments always start with a clear goal that the teacher can express, usually on the assignment sheet so that students understand the goal as well.
Good writing assignments also often take shape by thinking backwards. In effect, teachers ask themselves, "What do I want to read at the end of this assignment?" By working from what they anticipate the final product to look like, teachers can give students detailed guidelines about both the writing task and the final written product.