What is 'Groups' 's purpose?
Suppose you have a staging server. You have 20 users on that box but only 5 of them have access to modify files and whatnot in a certain directory (let's just call the directory "pre_live"). You give the permissions to "pre_live and all files/subfolders" as 775 (rwx-rwx-rx) and as group "staging". You put Joe,Jane,Mary,Steve,and Rick in group staging. That means now, those 5 people are the only ones that can write/modify files in that directory while all others can just execute/read the files. That's how groups work.