There are a number of ways to answer this.
First, we must take into account the fact that this was not intentionally planned as Pope Francis’s first trip abroad. The papal trip had already been scheduled when Pope Benedict XVI was in office. He was to go to Brazil for World Youth Day and Pope Francis simply took his place on the trip. In this sense, it really has no significance.
Second, we can say that the trip has significance because Latin America, and the “developing world” in general, is important to the Catholic Church. The Church is losing strength in the rich world and must now rely on members in places like Latin America if it is to maintain its importance in the world.
This brings us to a third point. The Church has been losing ground in Latin America to some degree. It has been under pressure from evangelical churches. Because of this, there is a felt need for the Catholic Church to pay more attention to Latin America since that has long been a stronghold for the Church.
Thus, we can say that the trip to South America is important because of the importance of that region to the Church and because of the Church’s desire to ensure that people in Latin America do not continue to leave for other churches.