"In the fifteenth year of the reign of Tiberius Caesar — when Pontius Pilate was governor of Judea, Herod tetrarch of Galilee, his brother Philip tetrarch of Iturea and Traconitis, and Lysanias tetrarch of Abilene — during the high-priesthood of Annas and Caiaphas, the word of God came to John son of Zechariah in the wilderness" (Luke 3:1-2).This story occurred during the period of time when the lives of seven different historical figures overlap. Why would someone waste ink and parchment on a detail like that?
Luke wanted to make sure we knew that the story he was about the tell is grounded in history. The story of Jesus didn't happen "once upon a time." It didn't happen "a long time ago, in a [place] far, far away." It happened "in the fifteenth year of the reign of Tiberius Caesar ...."
Why is that important? So that you and I can know the apostles "did not follow cleverly devised stories when we told you about the coming of our Lord Jesus Christ in power" (2Pet 1:16).
The Gospel is not just a nice story to tell sitting around the campfire; it is supposed to turn your life upside down. So it's important to know that this really happened.