The Undertaker has constantly been able to really mess with his enemies psyches by creating life-like dummies of his opponents resting peacefully in a casket. By doing this, he was able to truly creep out both his opponents and the audience watching in the arena and at home. This created the perception that the Grim Reaper-like Undertaker would indeed claim his opponents soul and make him 'rest in peace', giving him the advantage in the rivalry. I mean, who likes being confronted with the possibility of their impending death?! Everyone who was confronted with their dummy corpses did a great job of selling it. Kevin Nash, for example, looked genuinely terrified and when Undertaker did the same thing to the Ortons years later, their pretending to be freaked out was executed perfectly.