If you take up a study of the world’s ancient religions, learning about the early beliefs of India, China, Japan, Australia, and the Americas, you will find that their ancient heroes were mythical, not actual humans who lived in the old days.
Our religion is different. Jesus and Judas, Samuel and Solomon, and Joan of Arc and the rest were all real people like us. We can look up the approximate year when each of them was born and died. That allows us to see that the problems they struggled with were as real as what we must put up with.
Today's first reading, by giving the places and times when David ruled, gives us assurance that David was a real person they way each of us is, and it further makes him real by relating how the leaders of all twelve tribes came to him, accepting him as their king because they were “bone of his bone and flesh of his flesh.”