Guessing it isn't terribly useful.

You guess correctly. The issue is that POSIX states that

    ... all of the standard utilities [...] shall be implemented in a manner
    so that they can be accessed via the exec family of functions as defined
    in the System Interfaces volume of POSIX.1-2008 and can be invoked
    directly by those standard utilities that require it (env, find, nice,
    nohup, time, xargs).

Note however that /usr/bin/cd isn't completely useless: It has the same diagnostics (error message and exit code) as the command, so `env cd $FOO` is a way to check if you can change to that directory without actually doing it.

I too don't see how it is useful but it's certainly a thing. The Solaris implementation looks like this:

    #!/bin/ksh -p
    # ...
    cmd=`basename $0`
    $cmd "$@"

I just noticed that the what(1)-string (I haven't seen on of those for a long time) references "alias.sh", perhaps this is a clue?

    #ident  "@(#)alias.sh   1.2     00/02/15 SMI"

Were builtins actually aliases in an early shell? I still don't understand how this works though.

That's a funny implementation. It would end up being an infinite loop for any command that wasn't a builtin, or didn't have an identically named thing higher up the PATH. Like if you renamed it from /usr/bin/cd to /usr/bin/mycd.

On my Mac, it's

  #!/bin/sh
  # $FreeBSD: src/usr.bin/alias/generic.sh,v 1.2 2005/10/24 22:32:19 cperciva Exp $
  # This file is in the public domain.
  builtin `echo ${0##*/} | tr \[:upper:] \[:lower:]` ${1+"$@"}