The Liturgy of the Sacrament/Altar begins with the Offering, during which the altar is prepared for the coming consecration. The offering is the opportunity for the people to give of their own resources to the Kingdom of God. Recall, we are in the setting of a royal court and the act of giving items, or ”paying homage" to a king by giving items of physical value, is something which is common in such a setting. In so far as a king is the principle land owner then those who are vassals of the king are giving back to the king and kingdom a portion of what was produced by the king's land. In the same way, we are to consider ourselves stewards of that which ultimately belongs to our sovereign King and Creator. “The earth is the Lord’s, and all it contains,” the psalmist proclaims (Psalm 24) and St. Paul reiterates for the church (1 Corinthians 10:26). It is for this reason we speak in terms of giving back to God that which He has entrusted to us when we speak of giving tithes and offerings. "For all things come from you, and of your own have we given you" (1 Chronicles 29:13-14).
In some churches, at the outset of the offering you will see parishioners bring forward wine and bread. This harkens back to the period of time in which you did not simply buy prepared wafers and wine from the store but rather parishioners would bake and procure the bread and wine that was to be consecrated. The imagery is beautiful, for it reflects the feeding of the thousands, wherein the gift of one boy was multiplied by Jesus to provide for the needs of the multitude. In the same way it is from the gifts of the people that God feeds His people through the Eucharist. Notice that during the Prayers for the Whole State of Christ's Church the prayer is made that God would, "mercifully accept our alms and oblations." This is a reference to the gifts of the people that are given and prepared on the altar. When the Eucharist is celebrated without an offering collected thats line is read as, "mercifully accept our oblations," with the oblations referencing the wine and the bread given by the people for the purpose of the sacrament.
Although not universally practiced, the singing of the Doxology is a reminder of this reality. We are praising God from whom all blessings flow, because it is from His blessings so richly given to us that we can give back to Him and give to others as well. With the singing of the doxology the Offering is concluded.