Sometimes you might want or need to share a folder not only with groups, but also with additional users that do not / should not belong to one of the groups. Of course there is an easy workaround to set up another group just for this user, but it's not a clean approach and fills up the groups list.
It seems to be useful to have another option for r/w or r/o sharing with single users, although I know the name is groupfolders.