That allows the group leader to edit any groups for that member though. You can stop them adding to the admin group or editing admins but they could change ANY groups of any other member, not just the group they're supposed to be in charge of.
That's presumably because you haven't used it in phpBB. It's not in any other software than phpBB as far as I know, but when you have been using it for years and then it's removed, you see just how useful it was. When you have a website hierarchy, with inidividual areas of staff who have their own leaders for these areas, it's extremely useful for them to be able to edit the member groups that they are the leaders of.