Writing is not simply a form of assessment but an essential part of the learning process.
Frequent and well-designed assignments, formal and informal, promote the kind of active, critical learning essential to genuine mastery of course material.
Good writing assignments should be an integral part of course design, devised to accomplish the learning aims of particular courses.
Surface correctness (freedom from error) is only one characteristic of effective writing. Equally if not more important are conscious purpose, clear structure, cogent reasoning, and adequate development of ideas.
Writing is discipline- or context-specific, involving questions of audience, purpose, tone, structure, and format. Discipline teachers are able to provide the most relevant instruction in writing for their own disciplines.
Writing is a process, from the generation of ideas through drafting, revision, and editing -- the same kind of process faculty use. Assignments should be designed to encourage students to make use of the writing process.
Some writing is informal or "low-stakes" – done to gather and sort ideas, to respond to reading, to reflect upon work done: in short, writing done to learn rather than to demonstrate learning.
Teachers need not grade, comment on, or even read every piece of low-stakes writing they ask students to do.
Student writing—and confidence in writing—improves with practice, especially when assignments build in process and allow opportunities for revision.