I'm sure there are many examples of how this could be useful--here is one example:
A course's grades are divided into two main groups with associated weighting: Theory (60%) and Practice (40%).
Theory is divided into subgroups: Assignments (40%), Discussions (30%), and Quizzes (30%).
Practice is divided into subgroups: Research (50%) and Presentations (50%).
This is currently only possible by ensuring that your points for all assignments in a subgroup are properly "weighted", but that only works if course assignments are fixed, which means the teacher has no option of giving a pop-quiz or other flex assignment during the semester....at least not for points without lowering another assignment's points.
This would go a long way in improving Canvas Gradebook flexibility.