Group work can be effective for enhancing student learning in a number of ways:
Engaging diverse perspectives: Group work allows students to learn and grow from engaging peers' diverse perspectives and experience, challenging and enhancing each others ideas and assumptions
Jigsaw Content: When there is too much content for any one student to study in depth, divide the topics among various students. Then each student studies their assigned topic in depth and teaches group mates about it. That way all students encounter all the topics, but are responsible to dig deeply only in one or two. Students also tend to learn more deeply when they teach a topic to others.
Sharing the Load: Group work on projects and assignments can allow students to accomplish more than individuals may achieve alone. Group projects also allow students to fill roles that suit their strengths.
Learning to Collaborate: Group work helps students practice the important real-world ability to coordinate group effort, manage scheduling, leadership, followership, and mutual respect.