Yeah, you can easily forget which card was first and which was second.
The ordering can be done for strategic reasons for any drawing card or Caravan.
Ordering your cards matters whenever you have more actions and any draw coming. For some conditional draw, like farming village or spy, it becomes important to have things in the correct order. Likewise, loan and venture can keep drawing through green even with no more actions.
I think people are missing the point. Oracle draws two cards after reordering the top two cards, so it doesn't actually matter which one you put on top, since you draw both anyway.
Your opponents care - a cantrip in hand hand will draw one for them and not the other. You also care when you play other top deck attacks on their reordered deck.
Further, while currently no card cares about the order in which you draw cards, it is possible that future cards will care. E.g. Suppose there is a card in the final expansion that is something like VP-reaction "Reaction: You may reveal this card and your hand when you draw a curse. If you do and have no treasure in hand, you may trash the drawn curse."
Much like how the order of playing treasures never mattered until prosperity, I could see Donald keeping his options open for something like caring about the order of card drawing. It is much easier to be explicit on every possible permutation of play and implement that up front than to try to retrain players later.