In our professional careers, faculty members use writing every day for a variety of purposes:
to communicate information (memos, textbook evaluations, letters of recommendation, email)
to clarify thinking (when we work through an idea or problem in writing)
to learn new concepts and information (taking notes on reading and research topics)
Students need practice to be able to use writing effectively to meet these same goals. One or two writing classes just can't provide enough daily practice.