This is simply standard liturgical protocol.
The OFFERTORY of the Liturgy refers to the OFFERING of the Bread and Wine for the Eucharist–which historically is their actual placing on the altar, regardless of what other prayers or ceremonies accompanied this.
Since nothing else can be the matter for the Eucharistic Sacrifice, besides bread and wine mingled with water, nothing else can be or should be on the altar.
The Holy Oils are placed there since the are the matter of a Sacrament.
Wedding rings are blessed on the altar since they are the emblem of a Sacrament.
Perhaps tokens of other things, such as gifts for the poor, can be brought up–though not at the same time as the Eucharistic matter–to be blessed, but should NOT be placed on or near the altar, and should be removed immediately after this blessing.