Most of the time, our passports and diamond cards are all we need for check-in. But once in a while, we've been asked for our e-ticket receipts such as several weeks ago when we departed YYZ. So in addition to the reasons mention by earlier replies, this is another reason why it is a good idea to have them handy just in case. But no need to take them out and have them ready together with the passport & card unless they ask for it.