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

Controversies in the Vatican during Benedict XVI's term of office

Pope Benedict XVI gravely ill Pope Francis asks for prayers for successor of John Paul II

Pope Francis prayed for the health of Pope Benedict XVl

Benedict XVI, successor of John Paul II, passes away