Jesus proclaimed to humanity that ultimate death is just for sinners (the sinless won’t stay dead).
This page analyzes four evidences and their relevance:
This is relevant for two reasons: Jesus’s eschatological resurrection would simultaneously vindicate both the message of Jesus, and Jesus’s own sinlessness. (God certainly wouldn’t want it to seem like sinless Jesus wasn’t going to be raised after the ostensibly humiliating crucifixion which Jews thought was a lot for sinners; absent a resurrection, that could powerfully undermine the view of Jesus’s sinlessness and the Gospel. In fact, it did!)
Biblically, God communicated to humanity that ultimate death is just for sinners (the sinless won’t stay dead).
This is relevant because the Bible is reliable on this point.
Believers in Jesus would anticipate that their eschatologically resurrected bodies will resemble whatever Jesus's resurrected body was like.
This is relevant because not everyone would have been sure that a physical resurrection was a good thing.2 Pre-empting this, Jesus's resurrection body—with all its healthy and interesting power on display—would be overtly glorious visual proof that having a resurrection body is a desirable afterlife state to be in. The good news (Gospel) really is good.
While many of Jesus's miracles suggested that God was on Jesus's side, the physical resurrection of Jesus directly by God (without mediation) is a particularly flashy firework as far as miracles go. This is relevant because the resurrection therefore would be a particularly persuasive way to establish once and for all to Jesus's apostles (etc.) that Jesus was truly from God.