1. Writing about a subject helps students to learn that subject better.
2. Writing about content has special payoffs:
 -short-term: better writing
 -long-term: successful writers at higher levels of education and in life
 -better retention of content knowledge.
3. Content writing often motivates reluctant writers
4. Content writing develops language skills
5. Teaching writing teaches thinking.

from Teaching Writing in the Content Areas  Stephen N. and Susan J. Tchudi