about "forward-paragraph" in Intro to emacs lisp.

[waterloo]

I can not understand code of forward-paragraph in Intro to emacs lisp.

On " The forward motion `while' loop " of 12.4 in Intro to emacs lisp.
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     ;; going forwards and not at the end of the buffer
     (while (and (> arg 0) (not (eobp)))

       ;; between paragraphs
       ;; Move forward over separator lines...
       (while (and (not (eobp))     
                   (progn (move-to-left-margin) (not (eobp)))
                   (looking-at parsep))
         (forward-line 1))
       ;;  This decrements the loop
       (unless (eobp) (setq arg (1- arg)))
       ;; ... and one more line.
       (forward-line 1)
-----------------------------------------------------

Why need "(progn (move-to-left-margin) (not (eobp))) " ?
Why need the last line " (forward-line 1) " ? 

Thanks