What's the ".git/gitdir" file?

From: Kyle Meyer <kyle@kyleam.com>
Date: Oct 27, 2015, 22:04 UTC
Message-ID: <87a8r4ary9.fsf@kyleam.com>

Hello,

When a ".git" file points to another repo, a ".git/gitdir" file is created in that repo.

For example, running

    $ mkdir repo-a repo-b
    $ cd repo-a
    $ git init
    $ cd ../repo-b
    $ echo "gitdir: ../repo-a/.git" > .git
    $ git status

results in a file "repo-a/.git/gitdir" that contains

    $ cat repo-a/.git/gitdir
    .git

I don't see this file mentioned in the gitrepository-layout manpage, and my searches haven't turned up any information on it. What's the purpose of ".git/gitdir"? Are there cases where it will contain something other than ".git"?

Thanks.

-- Kyle git version 2.6.1