I'm a math teacher so we do a lot less group work than English/history/science classes, but I try to write at least one day of group work into my lessons a week. Here are some benefits.
1) Differentiation. This works two ways. First, you can build your groups to put students struggling in the same area together and target each individual group with the challenges that will benefit them most. The other way is to put students of varying success levels together. This is where people argue that the "slower" kids are holding back the "smarter" kids from succeeding. In my experience, that's false. By allowing these groups, you present the successful students an opportunity to view the material as teachers. It leads to a different level of analysis and understanding. The other side of the coin is that sometimes students are better at teaching each other. A student might be able to relate the topic to a classmate better than I related it in our lesson. I've seen a lot of growth from my students using both these strategies.
2) Classroom culture. Yeah, yeah, another one of those liberal ideas. Classroom culture's a real thing. If you have a happy class, you have a class that's going to succeed. A great way to build the culture in your class is to create relationships between your students, and group work helps with that. Students working towards a common goal.
3) Stress relief. The funnest part about implementing group work is creating fun challenges for the students. Letting them learn while they're simultaneously having fun is really something special. It also alleviates tension for the students. When we do individual work, there will inevitably be students who become stressed because they are struggling and are not good at dealing with the teacher one on one. When they're doing group work, they don't feel singled out and you still have the opportunity to give them that invaluable one on one time.
Obviously the teacher needs to take an active role in creating, planning, and implementing the group work. When done well, it works. I don't give a lot of home work, but on group work days I have my students take a survey for home work and design questions to give me the most revealing feedback. Adjusting based on that feedback is crucial. I think feedback is essential to creating effective group work, so if the teacher isn't taking feedback, something is wrong. To students who don't like group work, be honest but constructive with your feedback.