They do this pretty commonly; I think they do this to help distributed the crowds leaving the park. Basically, it just means the show won't start until 8:15 pm. After the show ends, you cannot get into line for any attractions and most shops and dining will be closed.
Rides take a while to dump their queue anyway, so the park is "open" past the close time. But essentially all ride entrances close, then WOC, then cast members form walls and slowly guide the WOC crowd out of the parks afterwards.