Luke is the only Gospel writer that got things in the order that they happened.  He went to the source for what he wrote.  He interviewed the eyewitnesses.  Otherwise we wouldn't have the stories about the birth of Christ.  He had to have interviewed Mary.

Elizabeth and Zechariah would have been very, very old when Luke talked to them because--they were both shocked when Gabriel said they would have a son.  Zechariah asked, "How can that be?" Perhaps Luke  got their story from Mary who was young when she went to see Elizabeth.  Mary was probably fifteen or sixteen. She wanted advice from her older relative.

Luke has stories that the other three Gospel writers don't have.  And he is the ultimate story-teller of the Bible.  He wrote about everything up to the Resurrection, and couldn't stop writing.  He continued his story about what happened next in the book of Acts. It is the only history book in the New Testament.

He was invaluable to Paul, who calls him, "Our beloved Physician."  Luke tells the story of sailing to Rome with Paul, and of what happened along the way.  He uses the words "We" and "Us."  No other writer does that.  Most likely, Luke was keeping a diary as he traveled with Paul, because the accounts are so vivid.  We can trace Paul's journeys through Luke's accounts.  We owe Luke a debt for sharing his travels with us.