A Fuller Definition of Writing to Learn
http://wac.colostate.edu/intro/pop4a.cfm

Although how writing fosters critical thinking is not clear (Applebee), theoreticians and practitioners alike agree that writing promotes both critical thinking and learning (See Adams, Britton, Bruner, Emig, Herrington, Knoblauch and Brannon, Odell, Parker on the linked bibliography.) As Toby Fulwiler and Art Young explain in their "Introduction" to Language Connections: Writing and Reading Across the Curriculum:

Writing to communicate--or what James Britton calls "transactional writing"--means writing to accomplish something, to inform, instruct, or persuade. . . . Writing to learn is different. We write to ourselves as well as talk with others to objectify our perceptions of reality; the primary function of this "expressive" language is not to communicate, but to order and represent experience to our own understanding. In this sense language provides us with a unique way of knowing and becomes a tool for discovering, for shaping meaning, and for reaching understanding. (p. x)
In "Writing to Learn Means Learning to Think," Syrene Forsman makes the same point, but she directs her attention not to a theoretical justification but a practical rationale for writing to learn:

As teachers we can choose between (a) sentencing students to thoughtless mechanical operations and (b) facilitating their ability to think. If students' readiness for more involved thought processes is bypassed in favor of jamming more facts and figures into their heads, they will stagnate at the lower levels of thinking. But if students are encouraged to try a variety of thought processes in classes, they can, regardless of their ages, develop considerable mental power. Writing is one of the most effective ways to develop thinking. (p. 162)
The Consequences of Writing by Robert P. Parker and Vera Goodkin is an especially good resource on writing to learn. Following a detailed discussion of the theoretical links between language (especially writing) and learning, these authors outline projects that focus on writing in entomology, clinical nursing, psychology, and mathematics, all with similar results: students learned key concepts and understood material more fully while also practicing some features of discourse for the specified discourse community. Thus, writing to learn can have additional positive effects in helping students mature as effective communicators even though the initial goal is to help students become better learners.

-abilene