Why are the bodies of deceased Popes displayed? Where does the tradition come from?

This is how Benedict XVI's body was brought into St. Peter's Basilica.

Pope Emeritus Benedict XVI arrives at St. Peter's Chapel

Who was Benedict XVI, the first pope to resign his office in 600 years?

The spiritual testament of Benedict XVI

When and where will Benedict XVI's funeral be held?

Pope Emeritus Benedict XVI dies at 95

Benedict XVI, successor of John Paul II, passes away