Timothy the Child in the Faith of Paul

Paul sent this letter to Timothy who he called "his child in the
faith" to encourage him and to give him orders. And Timothy is not the
only one who was called "child in faith" by Paul but some other
apostles were also called his child in faith or his beloved child. So
what is the importance of this "father son" relationship between Paul
and Timothy in this letter and other stories in the Bible? What do we
have to learn from this relationship?