What I imagined from your initial description is that you were doing the following:

  for (var i = 0; i < 3; i++) {
    await delay(1000);
  }

However (as you are possibly well aware), this line in your example is starting all the work immediately and essentially in parallel:

  const promises = Array(3).fill(null).map(() => delay(1000));

So the timing of your for...of loop is that the first element probably takes about 1000ms to complete, and then the other two seem to happen instantly.

Promise.all is just an alternative to writing the for...of await loop:

  await Promise.all(promises); 

I guess it relies on you already being familiar with the Promise API, but I feel that Promise.all() has slightly less cognitive load to read and its intent is more immediately clear.

A strong case for preferring the Promise.all() is that Promise.allSettled(), Promise.any() and Promise.race() also exist for working with collections of promises, and unlike Promise.all(), they would not be so easily reproduced with a one liner for...of loop, so its not unreasonable to expect that JS developers should be aware of Promise.all(), meaning there is no reason for it not to be the preferred syntax for the reasons I stated above.