Devotion to God through those people who have demonstrated love of God.
St Christopher (yes he’s still a saint) demonstrated love of God to the point of carrying the Christ Child AND the weight of all our sins while traveling across a river. So wearing a St. Christopher medal demonstrates our desire to emulate St. Christopher’s devotion to God and we wear it often while ‘traveling’ or being in a job that requires frequent traveling, such as truck driver, airline pilot, etc., because those professions are like his.
Medals of the Blessed Virgin Mary are popular because, especially for women, they help us remember how Jesus was conceived, born of, and cared for by His holy Mother. Those of us who are mothers or in ‘motherly professions’ such as nursing or teachers, would especially focus on loving our children and raising them to be like Jesus, and are reminded of this by the medals of Mary.
Musicians remember how God was glorified through music by St. Cecelia. Those who ‘doubt’ remember the ultimate faith of St. Thomas who said “My Lord and My God”. Those of us with saint names (and there are many) often enjoy the stories of the lives of “our saint” and how they loved and served the Lord --John, Paul, Adam, Robert, Michael, Gabriel, Peter; Agnes, Barbara, Theresa, Rose, Susanah, Judith, etc. Each medal, each saint, each story, ultimately lead to glorifying God and trying to become more Christ-like, loving, and trying to do God’s will.