"While the offerings are being gathered, the presiding minister or an assistant prepares the altar for the celebration of the Sacrament."
No, it's not a rubric to fill dead time; it's a rubric to confess the deep inner connection between the offerings being gathered and the bread and wine that will be offered to God - first confessed as His gifts to us - so that He will give them back to us as the Body and Blood of His Son. Bread and wine do not magically appear. We remember that in the ancient Church the faithful brought gifts of bread and wine - most of which went to the poor and the support of the clergy, but some of which the deacons set aside for the Eucharist on that day. From what the people gave, the earthly elements were set apart for offering to God's holy purposes. Nowadays, the people bring monetary offerings usually, but the principle is the same. Nor does He ask from us grain and grapes - which would be His gifts directly. He asks us to present bread and wine - that is His gifts taken and worked with human hands, and then offered to Him. You begin to get the pattern: it's an W not an M. It starts up at the top with Him, and includes a bounce up to Him of the gift received, which He then sends back as more than we could imagine, and then we send up thanks and praise and service to the neighbor. The W instead of the M keeps it from being idolatry. It begins always with the downward gift and that is where the accent remains - only each time He ups the ante and gives more than we can either desire or deserve.
So Sunday as the plates are being passed, remember that the offering is made by those who have received good gifts from God and who are about to witness the miracle of what happens when God's gifts gifts are taken and offered back in thanksgiving to Him. He to us. We to Him. He is to us. We to Him. This little rubric about preparing the table as the offerings are being gathered reminds us that this is the pattern of life with our Giver God.