Writing more frequently helps students capture ideas--images, sensory details, connections between ideas, comparisons/analogies, etc.
Writing, reading, and critical thinking are intimately related: we tend to know best the material we write about.
Writing more frequently helps students think like writers. (Think about the last time you tried to learn a new physical skill--skating, skiing, swimming--and how much more comfortable you got as you simply put yourself repeatedly into the physical environment for that activity.)
