POSIX and conventional Unix file system permissions are organized into three classes, user, group, and others.
The use of groups allows additional abilities to be delegated in an organized fashion, such as access to disks, printers, and other peripherals.
Unix processes have an effective (EUID, EGID), a real (UID, GID) and a saved (SUID, SGID) ID.
On FreeBSD, porters who need a GID for their package can pick a free one from the range 50 to 999 and then register this static allocation in ports/GIDs.
On many Linux systems, the USERGROUPS_ENAB variable in /etc/login.defs controls whether commands like useradd or userdel automatically add or delete an associated personal group.