What are the demented Windows8 fans doing here?
It's for developers so that the apps become better and they are willing to pay for it so everybody wins.
It's a landing pad for paratroopers who will wear the watch. They will demo the new phones and watches and show you the new find my iPhone feature that will now show in 3d along with your heartbeat, blood pressure etc. Siri will then guide them blindfolded to the landing point.
It's an obstacle course that will demo the watch and phones. Athletes will wear the watch and go through tasks to show what it can record and what it can withstand.
Could possibly be an outdoor stage for some sort of music concert they have planned, an athletic facility to show off the capabilities and health features in the iwatch, or it could always be that they're building an onsite store? Maybe they're releasing the phone early for people in attendance. Guess we'll have to wait and see.