It's not about the icon per se. It's about immersion.
It's about feeling excited beyond the knowledge that you're going to a cool halloween event and actually stepping into a story. If I had it my way the hype would start early in the year in the form of an alternate reality game (think 2008's legendary truth) that worked it's way slowly to a reveal. It's just that much more exciting when you actually feel involved and connected to the event on a personal level. When you can suspend your disbelief just that much easier.
I realize that not everyone cares about these kinds of things, so a balance is hard to strike. You don't want to make the casual goers feel locked out of the loop by the time the event rolls around. Thus, the designation of an Icon with accompanying theme is usually the most efficent way to establish immersion.