Luke 1:2 Even as they delivered them unto us, which from the beginning were eyewitnesses, and ministers of the word; 3 It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus, 4 That thou mightest know the certainty of those things, wherein thou hast been instructed.
Luke, a disciple of Peter, writes this gospel for us. It is Peter's account of the gospel which he relates.
Peter was the most intimate of all the eye witnesses. He was part of Jesus' inner circle, along with James and John. He seems to be the preeminent leader of the apostolic band. Luke quotes him as saying, "It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order..."
Now, Peter had as good of an understanding of the story as anybody. He was with Jesus from the first, that is, he was one of the first apostles called in the beginning of Christ's ministry. But his "perfect understanding" wasn't from the first. In fact, he never really seemed to "get it" until after the resurrection.
Indeed, Jesus told Peter, "after thou art converted, strengthen the brethren."
He was constantly misunderstanding or speaking before he thought. He overstated his own commitment. He understated the blessing of what he witnessed at the transfiguration. He brandished the sword when he should have kept it sheathed. He rebuked the Lord for predicting His passion. He faltered as he attempted to walk on the water. He denied Christ in the courtyard. Peter made more mistakes than any of the others.
But the light of the sun on resurrection day that revealed an empty tomb also illuminated his understanding of the complete story.
Prophecies, birth, life, ministry, teachings, death, resurrection, return, reign.
You have to have all of these or the story is not complete. By the time Simon Peter would hang up-side-down on a cross in Rome, he had a perfect understanding of all things.
Hence, the book of Luke.