Why? Electric Phred notes a report that says: "Sidewalks on two sides of the venue are inaccessible to pedestrians. Enormous power generators are stationed on the Larkin Street side of the building, occupying a lane of parking spots."
This leads him to this conclusion: "What sort of unveiling needs a 7000 seat venue, weeks of work, and likely millions of dollars? This wouldn't seem to me to be just the unveiling of the iPhone 6s or the iPad Pro (which one rumor has sporting an Intel Processor). I am betting we may see the Apple electric vehicle. Since the rental on the venue seems to stretch past September 9, perhaps it will turn into a mini Apple trade show and fans will be able to oogle and touch various types of unobtanium like the car, the iPhone 6s, the iPad Pro. Perhaps we'll see a Mac book, which some think may be equipped with the new 3DXPoint memory developed by Intel and Micron Technology. Certainly there is now a precedent of having unveiled the iWatch which was then first delivered weeks later."
Admittedly, I have no idea why Apple is holding the media event in such a big venue. Though Apple Car rumors are picking up speed, my crystal ball says there's a snowball's chance in Hell of such a vehicle being driven out on Sept. 9. (That said, I'd love to be wrong, as it would give us Apple journalists LOTS of material to write about.)
Also, I don't foresee any Macs being introduced at the event. I'm predicting new iPhones (practically a given), a next generation Apple TV, and perhaps new iPads (including the long-rumored "iPad Pro"). Apple will likely announce release dates for iOS 9 and OS X El Capitan. A much longer shot is an Apple TV streaming service.
If — and I emphasize IF — Apple is indeed planning its own vehicle, I don't think it will debut before 2020 at the earliest. And, yes, Apple announced the Apple Watch a few weeks head of availability, but I can't see the company announcing a product four years in advance.