None of us learn best passively absorbing knowledge from the teacher and text.
Group work provides students with opportunity to practice conversation.
The students bring experiences, insights, and voices to this social discourse that, if properly orchestrated by the instructor, can help students write. This must be taught, and then students must be held accountable for participation. Grades, rewards, the extrinsic motivators will pull in more of the non-participants. Peer influence is a strong force in the classroom. Teachers need to harness this energy and pull in the disengaged wallflowers.