The current option (deleting all groups and manually add one by one + adding these to users again) is a an hassle, because you need to know what groups an user had. And lastly, you need to reconfigure permissions again.
How It Works?
When Re-arrange Groups mode is enable, you can change the order of groups without needing for doing the steps stated above.
Why It Will Be Useful?
This feature should be useful if you want to arrange your groups nicely without the need to do the nuke-add-adjust-add people method ever again.