Chap 2.3 The purpose of add "addUpdateListener" for "alphaAnimator"

When I study Chap 2.3, I’ve got a question.

When declaring alphaAnimator, we already set animation for alpha at progressBar.

val alphaAnimator = ObjectAnimator.ofFloat(
  binding.progressBar,
  "alpha",
  0f,
  1f
)

But we add updateListener for alphaAnimation and set alpha again.

alphaAnimator.addUpdateListener {
    binding.progressBar.alpha = it.animatedValue as Float
}

As I understand, “the behavior of adding updateListener” and “the behavior not adding it” are the same.
What I understand is right?
Is there any strength for adding “updateListener”?