Isso é explicado na novela de 1989 do filme . Em suma, eles estavam lá em cima para ter uma "visão melhor" da briga que acontecia abaixo (por exemplo, eles estavam se escondendo)
He reached past Vicki and pushed open the trapdoor. There!
“Upsy-daisy,” he said to Vicki as he gave her a final shove up into the belfry. As the Joker followed her up, someone said, “Hiya boss.”
Three men were waiting for them—the three bozos he had hired from that Kung Fu studio. He had never learned their names, so that’s how he thought of them—as Bozo One, Bozo Two and Bozo Three.
“What brings you here?” the Joker asked the bozos. “Getting religion?”
Bozo One, a lithe, medium-size man who wore trick boots, said, “I was getting confused down there.”
“We come up to get a bird’s eye view of what was goin’ down,” said Bozo Two, a slightly larger, heavier man.
Bozo Three, a huge black man whose face was covered with tribal scars, said nothing. The Joker thought of Bozo Three as the most dangerous of the lot.