and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord (Rom 1:4)
τοῦ ὁρισθέντος υἱοῦ θεοῦ ἐν δυνάμει κατὰ πνεῦμα ἁγιωσύνης ἐξ ἀναστάσεως νεκρῶν, Ἰησοῦ Χριστοῦ τοῦ κυρίου ἡμῶν (Rom 1:4 BGT)
Union with Christ is the description of the reality that believers are united with Christ and share in all his benefits. It is not a step in the Ordo Salutis, the logical ordering of the steps by which God accomplishes and applies the benefits of Christ to us who believe, and which can be seen in part in a passage like Romans 8:28-30. Rather, Union with Christ describes the entire reality of Christ's work for the believer, from election to glorification. It is a descriptive term, not a procedural or action term. For as an example, we are united with Christ, in his burial and resurrection, in baptism (Col. 2:12), a glorious truth that describes what happens during our baptisms, with the action word being "baptism," not "union."
With that said, the descriptive truth of Union with Christ is the thing that bridges what God has done in Christ, with what we ourselves benefit from Christ. The whole of Scripture is about Christ (Lk. 24:26), not us. Therefore, how does the Scripture aid us? It aids us through the fact of union, because then in union with Christ all of what Christ does is done and given for us and our benefit.
How does Christ's resurrection, Christ's kingship, benefit us? One way of answering this question is to speak about how the atoning work of Christ merited salvation for the elect, through our justification, adoption, sanctification and glorification. Another way to speak about it is the biblical-theological manner of union. In Romans 1:4, Jesus was declared to be the Son of God in power ... by his resurrection from the dead. Christ's resurrection is not just done because a person who is God cannot actually die (which is true). But Christ's resurrection also is the declaration of His Kingship over the world. Romans 1:4 is not after all talking about God becoming the Son of God in power after his resurrection, because Jesus was already God the Son and the Son of God prior to his death and resurrection. Rather, in Romans 1:4, Jesus is now becoming the "Son of God" in a new sense, as King of the cosmos by merit (rather than by right as God). As the God-Man, he now sits enthroned as King over all, both as Creator and now also as representative of creation.
As the representative of creation, Jesus in His humanity is king. Being united with him now, we also partake of the benefits He has merited for us. In the King's victory, we are victorious. In the King's glory and riches, we share in that especially at the end of the ages. And through this union, we partake of all of Christ's benefits as we submit to our King.
Most of the time, we focus only on Christ's death on the cross, which is important. But Christ's resurrection is also important, in its own way. He is resurrected, for us, and thus as we look and meditate upon this truth, we can be assured that Jesus is our King who merits all good things for our good. Amen.