They begin to collaborate in problem-solving. Better now because communication about the subject has improved. They learn that mathematics is really used for something. They learn to use their intuition and common sense about attacking a problem. They learn that not all problems have simple answers or answers at all.
Good students get better when they take on leadership roles in groups.
Weak students can become weaker as they perceive the others strengths. Confidence that they know as much as partners and that there is always something positive that they can contribute. Many lost their math anxiety and were anxious to take more classes even though they had dreaded this course. I teach a lot of 100 level courses. For these students, the most obvious benefit is the patience they develop in working on multi-step problems.