what is 'nogroup' group's purpose

nogroup is the group analog to the nobody user. It is used for unprivileged processes so that even if something goes wrong the process does not have the permissions to cause any serious damage to an important user or group.

Debian's documentation for nogroup explains it as:

nogroup (user: nobody): Daemons that need not own any files run as user nobody and group nogroup. Thus, no files on a system should be owned by this user or group.