They gain self confidence and are willing to attempt problems that seem hard. The positive gain I have observed from students in learning mathematics and using cooperative learning is the fact that I hear and see more students in my classes discussing mathematics and asking more questions. Informally, I believe that my students gain a deeper understanding of mathematics and a more realistic understanding of how mathematics is done through the use of cooperative learning. I'm not sure about increased learning, but I am sure their attitudes are improved. Students learn that they can master math without my pouring it into them.
Students learn to discuss mathematics.
They become aware of multiple approaches of solving a problem. They learn to work longer on a problem. Homework done as described in question 9 above has led to a significant improvement in the students ability to make clear written presentations of their mathematical thoughts. Students who have had to make presentations of new material to the class have a much better understanding of that material than if they sit and listen to me lecture. Fragile students benefit the most.