[plt-scheme] Question chapter 10 box issue

Hey I have a problem with box.We wrote function called
cyclically-bind-and-interp i understand that function.I mean we have to
create cyclic environment for doing some recursion but i dont understand why
we use "box".Function like that.
(define (cyclically-bind-and-interp bound-id named-expr env)
  (local ([define value-holder (box (numV 1729)) ]
  [define new-env ( aRecSub bound-id value-holder env)]
  [define named-expr-val (interp named-expr new-env)])
  · · ·))


some who knows box issue,can explain me thanks.
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