F.4.1 Why does Red Hat `adduser' put each user in his/her own group?

It's called the user group scheme, and it's described in the manual. What it basically does is allow you to use groups to allow multiple people to use a common set of files and edit them, transparently. It goes like this:

• You'd like to have a group of people work on a set of files in say, the ``/usr/lib/emacs/site-lisp'' directory. You trust a few people to mess around in there, but certainly not everyone.
• So you do ``chown -R root.emacs /usr/lib/emacs/site-lisp''. And you add the proper users to the group.
• To allow the users to actually create files in the directory you do ``chmod 775 /usr/lib/emacs/site-lisp''.
• This is fine, but when a user creates a new file it gets the group of the users default group (usually users).
• To prevent this you do `chmod 2775 /usr/lib/emacs/site-lisp' which causes everything in the directory to get created with the ``emacs'' group.
• This is fine, but the new file needs to be mode 664 for another user in the emacs group. To do this you make the default umask 002.
• Well, this all works good, except that if your default group is ``users'', every file you create in your home directory will be writable by everybody in ``users'' (usually everyone).
• To fix this, you make each user have a ``private group'' as their default group.

At this point, by making the default umask 002 and giving everyone a private default group, you can easily set up groups which users can take advantage of without doing any magic. Just create the group, add the users, and do the above chown and chmod on the group's directories.