The purpose wasn't pure cruelty, but was intended to display the awesome power that Rome possessed over its enemies. Rome used this feature of the procession as a means of reassuring its citizens and of intimidating its enemies. It's not surprising then that St. Paul, himself a citizen of Rome, would choose this familiar concept of the Roman triumphal procession to describe what occurred on Easter morning. Think of Easter worship in terms of the Roman triumphal procession. Jesus is the focus of our worship as he receives honor and accolades for conquering the enemies that men fear the most.
Easter is no small victory.
Death, hell, and Satan are very powerful enemies that have held the whole world hostage ever since Adam and Eve's fall into sin. These great enemies of mankind kept us captive in fear all of our lives up until the time we came to believe in the living Christ. Before Easter we were powerless to escape death, for we were already dead in trespasses and sins. Hell threatened to swallow us up. Satan accused us before God.