Unity - IEnumerator output returns null

Question

I am currently trying to understand IEnumerator and Coroutine in the context of Unity and am not too sure that it is performing a "yield return null". At the moment, I believe that it basically pauses and waits for the next frame, and in the next frame it will return to execute the while statement again.

If I omit the "yield return null", it seems that the object will instantly move to the destination, or perhaps "skip a lot of frames." So, I think, my question is how is this "yield return null" function inside this while loop and why is it needed.

void Start () {
    StartCoroutine(Move());
}

IEnumerator Move(){

    while (a > 0.5f){

        ... (moves object up/down)

        yield return null; // <---------
    }

    yield return new WaitForSeconds(0.5f);

    .... (moves object up/down)

    StartCoroutine(Move());
}

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