• you can edit any instances on scene, not just the ‘source’ one
• you can work in recursive way – group instances, can be made of other subgroups, which can be made of subgroups etc. At any stage subgroup can be edited and saved, with all instances updated in real-time
• easy mirroring of selected groups (fixes group instance mirror bug in blender)
• adjusting origin of group, without destroying positions of other instances placed in scene, or in another groups
  • ability to edit linked libraries groups directly in current blend file
• ability to crate group proxy

This makes working with huge scenes with lots of objects easy and gives all the pros of group instances:

•  less memory memory usage
•  easy selection of multiple objects with one click