I've said that we like to think we write immersive games. This is reflected in the writing style. We generally choose a game that we can picture. We run the things we imagine to be cool (Who doesn't?). We will be looking for visual moments.
We ran an event called Dark Hearts last year. It was a player event set in the PD Empire game world. We wanted the players to be in a small enclosed village. We wanted them to know the outside was bad. The event was written around a series of images from fairy tales. We wanted a little ghost girl in a red cloak walking around the village. We wanted an old hag. We also wanted moments. We wanted them to be out in the dark doing rituals and summoning an eternal. We wanted them to have to cut a body open. We took those elements (and more, which didn't make the final cut) and wrote an event around them.
Moments lead to Story Arcs. Every event has them. They can be short things that last an hour or so, or, in campaign systems, they can last for years. Over time each arc (or thread) will involve a varying number of people. We combine several arcs into each event, with some picking up before others drop off to keep people involved in things.
Knowing how many players we have helps here. We want to be able to occupy our players throughout. We know roughly how many players can be involved in each arc at any one time. The total across all the arcs should add up to the right number of players (allowing a bit of leeway for eating and sleeping times - but really - who goes to a Mandala event expecting to sleep?).
Moments and arcs build the initial structure of our events. We do also use them when writing plots to run at larger games. Moments really come into play there. Moments are something we're always exploring. We hear them from everyone. People constantly talk about what they want to see at an event, and often that involves moments, be it the players arguing about where to send an army, a wave of heavily armed organised monsters charging down the a slope towards a battle line or players sparring in the middle of camp while children play around them.