The Transfiguration happened so that Peter, James and John would persevere in hope. It took place on the way to Jerusalem. The glorious sight on Mount Tabor confirmed their conviction that Jesus was from heaven. They could see that He was at least equal with Moses and Elijah. Although they might have had an inkling of His Divinity, they would not articulate that until the after the Resurrection. The Transfiguration helped ensure that the three Apostles would be around long enough to profess the Resurrection.
Peter especially would need help avoiding despair. At the Last Supper Jesus even prayed for him in particular that he would be in a position to confirm the brethren after those horrible events leading up to the Son of Man being hoisted between two thieves.
After the Resurrection, having professed the Divinity of Jesus, the Christian can revisit and reflect on the Transfiguration, realizing that Our Lord’s Divinity was being revealed. Remarkable, isn’t it, that Jesus’ human features need not be obscured for His Divine Nature and Eternal Sonship to be manifest?
What also has been transfigured, we realize, is the human face itself not just Christ’s. The human face reveals the divine, whether it be the only-begotten Son of God or the adopted sons and daughters created in the image and likeness of God.
The Transfiguration is so much more than a blinding spectacle that left the Apostles bewildered and babbling. It touches the mystery of God made flesh.