Teachers are facilitators in the classroom. Students should grow independency and they should be able to work independently of the teacher. When students work in a group, they are free to talk and listen to each other, work on problem solving situations and get to know their peers. It is also important for them to lear how to respect their friends. They have the chance to be responsible and help each other.
Here is another video from youtube ( I think youtube is an amazing source), I hope you enjoy it.