The best reason for a big event...
is being big. Nah, HUGE. Ordinary big isn't good enough any more.
Big events, grand openings, national events that just can't be missed. These work (if they're big enough).
Big events, if they're truly big, change the rhythm and demand a different sort of attention and preparation. We can push through the dip, expend emotional labor and do things we never thought we'd be able to do if there's a charette and a deadline and an audience.
Human beings respond to emergencies and to hoopla. We like doing what others are doing, and we'll suspend social disbelief if we're being carried along by the pack (or the mob).
The challenge comes when we institutionalize the event and make it normal.
If you're going to have an event, better make it big. Or even bigger than that. It needs to be awe-inspiring, frightening, on deadline and worth losing sleep over.