buffer variable values

[drain <aeuster@gmail.com>]

Original insert-buffer function:

(defun insert-buffer (buffer)
  "Insert after point the contents of BUFFER.
  Puts mark after the inserted text.
  BUFFER may be a buffer or a buffer name."
  (interactive "*bInsert buffer: ")
  (or (bufferp buffer)
      (setq buffer (get-buffer buffer)))
  (let (start end newmark)
    (save-excursion
      (save-excursion
	(set-buffer buffer)
	(setq start (point-min) end (point-max)))
      (insert-buffer-substring buffer start end)
      (setq newmark (point)))
    (push-mark newmark)))

Under what conditions would (or (bufferp buffer) not be nil?

In the Emacs Lisp Intro, in the discussion of this function, there seem to
be three different "buffer" variable values referred to:

(assuming "web.org" is an existing buffer):

(1) #<buffer web.org> 
(2) web.org
(3) web

(1) is what setq assigns "buffer" in the expression (setq buffer (get-buffer
buffer)).
(2) is the name of an existing buffer that the interactive expression will
permit the user to enter.
(3) is a string that refers to no buffers.

I don't understand the point in the OR expression. How would the user pass
as argument to the function a buffer, rather than just the name of a buffer?
Entering #<buffer web.org> is invalid. 

If this function did not read input from the user, the OR expression would
serve a purpose; since "buffer" could have been assigned an actual buffer
elsewhere in the body of a larger function (that insert-buffer had been
embedded into).




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