Writing is a skill that all businesses want their employees to be able to do well. Teachers in all content areas must add writing to their content courses. In so doing, they not only enhance their students' writing ability, but they also increase the understanding of the content and the vocabulary of the discipline. Teachers can use writing as a classroom assessment for learning as they examine students' explanations of what they are learning. (from the West Virginia Department of Education)
Following the above hyperlinked text will get you to some great resources for using writing across the curriculum.