Explicit delay in flatten-stream

Why does flatten-stream use delay explicitly? What would be wrong with defining it as follows:

(define (flatten-stream stream)
  (if (stream-null? stream)
      the-empty-stream
      (interleave
       (stream-car stream)
       (flatten-stream (stream-cdr stream)))))

---

This exercise has no tests. Any solution is a right answer.

Run