I'm guessing it was there mostly to show off pretty CG effects and how powerful she was. Yet I thought the x-gene, y'know, came to be only during puberty? And the whole idea of either group being able to so easily destroy the home without anyone else in the neighborhood calling the cops or something is just stupid.
That is actually one of the most accurate scenes in the movie, continuity wise. In the comics (and the novelization) Jean as playing outside with her best friend Annie when Annie is struck by a car. As the driver flees, Jean instinctivley uses her power to go into Annies mind to try and save her. In the comics, Jean barely makes it out of Annies mind before she dies, and is rendered comatose for a short time before Xavier pulls her around.