The great second part of the Mass is called the Liturgy of the Eucharist.
It begins with the preparation of the altar. During the first part of the Mass the altar serves only as a temporary, central “throne” for the Book of the Gospels. By now the Gospel book has moved to its more permanent throne near the ambo, the place where the Scriptures are proclaimed. Although a large cloth covers the altar’s entire table-top and candles may be placed upon it, other than these things the altar should be bare. It is not a place for things to be “kept until needed,” much less a place to be cluttered with papers, matchbooks, or worse. (When I was growing up, the pastor of my home parish emphasized this as much as possible. He’d really get mad if you accidentally put something on the altar that didn’t belong there. He even had a special gold cloth cover for it. The altar servers immediately after Mass went to put that cover on the altar to remind people that it was a special thing, and not just an ordinary table. Not a bad idea, when you come to think of it.)
Anyway, when the priest and assembly sit after the Prayer of the Faithful, only then do assistant ministers bring a few more necessary items to the altar: a cloth “corporal” and “purificator” (think of them as a place-mat and a napkin!); the principal chalice (cup) which will be used; and the Roman Missal (the book of texts used by the priest). Have you noticed this when it happens?