I think I'd say that in preaching, the preacher enters into the work that God the Father is doing through the Spirit of Jesus in the midst of the congregation. In fact, my only confidence in preaching comes from the fact that "as men of sincerity, as commissioned by God, the sight of God we speak in Christ" (2 Cor 2:17). United to Christ by the Spirit, entering into the apostolic and priestly work that Christ is already doing, sent by God to a particular people to proclaim his Word in his presence, I preach.
And so, to begin to answer the question about what is preaching's goal, we have to say that the goal is to enter into the work that God is already doing in the midst of his people for his glory and for the extension of his reign in their midst and in their place. Only in this way will the Gospel come not merely as word, but as Spirit and power (1 Thess 1:4).