Why do I need to check if a liquid-fire transition has a newElement?

The default liquid-fire transition checks if this.newElement exists.

import { Promise } from "liquid-fire";

// This is what we run when no animation is asked for. It just sets
// the newly-added element to visible (because we always start them
// out invisible so that transitions can control their initial
// appearance).
export default function defaultTransition() {
  if (this.newElement) {
    this.newElement.css({visibility: ''});
  }
  return Promise.resolve();
}

When would this.newElement not exist? Isn't it a guarantee?