Why in the new order is it thought to be so important to celebrate the "liturgy of the Word" from the sedilia rather than the altar, as in the TLM? Even amongst his mini-rants against some of ways in which the NO has been said, (at that time) Card. Ratzinger in his "The Spirit of the Liturgy" remarks that the shift to the sedilia was an important one. Why? I don't understand that.
I honestly cannot comment on the mind of (then-)Cardinal Ratzinger when he speaks of the importance of the shift from the altar to the "presider's chair" with regard to the Collect, but my own observation is that it has to do a great deal with a re-focusing and a re-purposing of the attention given to the altar. The New Order of Mass (Novus Ordo Missae) of Paul VI--and what continues to be the prevailing theology in even the most recent documents--places particular emphasis on the Altar of Sacrifice as that aspect of the church building that has but a singular purpose: it is the locus on which the unbloody Sacrifice of Christ on the Cross is re-presented for the People of God in the Eucharistic Supper of the Lamb. To that end, those actions which have no direct and immediate correlation to that Sacrifice (such as the Collect, bidding prayers, the Liturgy of the Word, etc.) are seen only to distract from that singular and all-important function of the Altar of Sacrifice. [NB: this is my own estimation, and not a dogmatic declaration.]
The emphasis of certain actions and prayers of the Mass taking place at the sedilia (or "presider's chair," depending on your preference) is not one of de-emphasizing the altar in favor of the chair, but rather of clarifying and streamlining the focus of the altar. The current General Instruction of the Roman Missal (GIRM) makes it very clear that the chair should be in a prominent place, but should in no way resemble a throne or a place of such importance so as to distract from the altar itself.
An examination of the GIRM also makes it clear that those prayers which are directly associated with the sacrificial nature of the altar (e.g., the Post-Communion prayer) may be prayed at the altar:
Then, standing at the chair or at the altar, and facing the people with hands joined, the Priest says, Let us pray; then, with hands extended, he recites the Prayer after Communion (GIRM 165).
This is by no means a comprehensive answer, and I have no doubt that any number of my more traditional confreres will jump at the opportunity to suggest that this ought not have been changed. However, as faithful sons and daughters of Holy Mother Church, it is incumbent upon us to accept those which which are given to us for our edification of our souls and the renewal of our worship. The logic in the decision to remove from the altar those aspects of the Mass which are not directly and immediately connected to the action that takes place on the altar is not necessarily a bad one. Whether the theological or liturgical principles I stated above came into the decision is beyond me.
[Personally, I think it far more likely that this adjustment is the result of a reductionist attitude by the reformers to make the average parish Mass align more closely to the style and rubrics of the Pontifical Mass of the Usus Antiquior (Extraordinary Form), in which the bishop would pray the prayers from his cathedra, and not from the altar.]