The responsory from the Office of Readings for today, the feast of St Luke, reveals something of note:
Having first traced it carefully from its beginnings, Saint Luke wrote his gospel,* that we might understand the instruction we had already received, in all its certainty.
He wrote concerning all that Jesus set out to do and teach,* that we might understand the instruction we had already received, in all its certainty.
The response - that we might understand... is the key. St Luke studied all that was being said about Jesus of Nazareth (mainly from St Paul, whom he was travelling with), and codified that Jewish focused Good News into a narrative that would reveal the Good News to the Gentiles. Luke includes a lot of detail about the Jewish world and faith that gentiles would not have been particularly aware of that they might understand where Jesus was coming from. No longer was God's covenant only with the Jews, but was for all.