Question

why does father have to tell stories to jo in the evening and saturday naps?

Answer 1

Because to join the evening snaps father have to tell stories about the world of

Answer 2

Author of the story John Updike's mother had her own literary inspiration when Updike was young and it was due to her that the books were a huge part of the author's life, which later inspired him and paved the way for him to pursue writing as a career.
Updike is known for mirroring his own life into his works and perhaps this special storytelling time between Jo and his father reminded him of his own relationship with his parents. The author probably put the storytelling angle to cherish his own days when his mother or father would tell him a story when he was young.