Seemingly no reason aside from idle amusement. Childermass isn't typically prone to that kind of entertainment, but it's nice to have one up on one of the greatest magicians of their time for once. He intends to enjoy it while the moment lasts.

"I can certainly do that," he'll at least give Strange that much. The trick is that he only asked to be shown it, it never mentioned anything about coming back afterward. So this time when he steps back into the shadows of the trailer, gathering them around him not unlike how his original trick with shadows before had worked, he'll vanish off to who knows where else in the carnival and he won't be back, at least not in the trailer.

Where he does end up is in the Big Top, emerging out of a shadowy corner near where he knows at least one mirror prop is kept for the sake of fairness. It wouldn't be any fun if he went somewhere that Strange couldn't follow.