The reasoning behind this statement is that when children help each other to learn they go through an intellectual process which clarifies their own thinking and understanding.
Mixed-ability groups are not only very good for 'peer tutoring' and helping the tutee but are also very good for helping the 'tutors' clarify their own understanding. In this way the more able children benefit as well as the less able tutees.
The first step on my own journey of understanding teaching came after five years when I was awarded a student teacher for six weeks. In those days the University did all the mentoring and class teachers like myself regarded it as a bit of a holiday! The children were quite challenging and the student's first question after his week of observation was: 'How do you do that?' He meant getting the children to sit, listen and do as they were told. After reflecting, I said I didn't know, I just did it! Pathetic, but true; however, it did begin my journey as a reflective teacher and did help me discover the advantage of peer tutoring to clarify my own understanding of teaching.
It has been my experience working with numerous teachers who mentor students that their professional knowledge is often tacit, but just like children who tutor their peers, once they begin to explain their teaching to others, they gain insights themselves. This is one of the advantages of mentoring students that has been mentioned by head teachers when I have carried out school placement evaluations, e.g. 'It helps my teachers to examine their own practice.'
The above is a rationale and logic for the intelligent use of group work that will depend on the curriculum content; abilities and temperament of the children; availability of resources; time constraints; physical classroom constraints etc. The important point is that it is your decision how and when you organize your class, from direct teaching to cooperative group work. The judgement is in balancing how you expedite the learning process in the most effective and manageable way.