scheme question

Hello /prog/, I'm not really much of a programmer but I've been dicking around with Lisp/Scheme lately for fun (DrRacket specifically).  One of the programs in my book goes like this:

;; move-circle : number circle  ->  circle
;; to draw and clear a circle, translate it by delta pixels
(define-struct circle (center radius color))

(define (move-circle delta a-circle)
  (cond
    [(draw-and-clear-circle a-circle) (translate-circle a-circle delta)]
    [else a-circle]))

draw-and-clear-circle draws and clears a circle, obviously, and translate-circle simply modifies the given "circle" structure such that its position is shifted by "delta" pixels.  So if I wanted to make a circle move across the screen I could run something like

(draw-a-circle (move-circle 10 (move-circle 10 (move-circle 10 (make-circle (make-posn 30 30) 45 'red)))))

Anyway my question is simply this: why is there an "else" at all in that conditional?  The function "draw-and-clear-circle" always returns a value of "true", or, if there's something wrong with the definition of the circle structure, an error.  So afaik the "else" would never come into play.

>>34
nearing your male occupation.
Actually, four out of the four female scientists/programmers (approximate ages 20,20,25,35, two white, one asian, one black) I know enjoy sexual/scatological/"offensive" jokes at the workplace and are very displeased with "easily-offended, socially-networked Feminists", much like their male counterparts. They regard them with much disdain, and qualify them as loud, manipulative, useless wastes of air (and I'm holding back, I've actually heard so much worse from the females than from the males).

This isn't about male vs female, this is about sciences vs humanities.