To the person on the street, it looks like a spontaneous event. But the same PR firm that does First Night did the circus. They came up with the idea of the parade to raise interest in the circus. It was dollar-driven. It's a constant battle.
Ephemera have profound influence on the city.
Yes, the Palio is actually a horse race between neighborhoods. It starts the evening before with huge banquets in the neighborhoods that are participating. They hire jockeys and strike deals about who would win. It's much more complicated than just a race that happens once a year. The impact isn't just in built form.