I love seeing global shutter on this camera. It's been amazing getting shots with cameras with global shutter for me.
Unlike film grain, the weird artifacts for sports/action, camera moves, rotary motion... it's all bad looking and disappears completely with global shutter.
Not just global shutter, but shutter speeds up to 1/80000, flash sync the same (which is simply bonkers, but possible due to global shutter), and pre-recording?
I love my A7IV, but the A9III makes it look like a toy.
While I love ridiculous specs as much as anyone, let's remember that 1/80,000th of a second is.. nuts and as far as I can find there's only one flash unit that can even do that, the profoto pro-11, which is a $20k proposition before you even buy the damn camera. And you will get very, very far from its maximum output for that kind of duration.
So it's very cool, yes, but unless you have a ton of money to spend and a very very good reason to need such extreme speed.. you're never gonna use it.
I am certainly looking at my trusty but aging a7sii with an increasingly contemptuous eye, though...
Flash doesn't have to be faster. Max flash sync speed means up to this speed the shutter is guaranteed to be fully open when exported flash signal triggers the flash. Below this speed is totally fine, above this could result in partially lit images.
e.g. A camera's max sync speed is 1/60s, and you connected a flash unit over sync cord, and set the camera to 1/250s, ignoring the lightning bolt sign on the dial. You take a picture, and alas, top half/left half is pitch black and only bottom half/right half is shown correctly, because you exceeded the max speed and mechanical circuit to trigger the flash did not close until image was already partially taken.
Or, e.g. A camera's max sync is 1/80ksec., with a random flash unit on top. You set it to that 1/80ksec, push down the shutter, the camera triggers flash, flash starts discharging energy onto Xenon gas tube. As the light intensity goes up, the image processor on the camera pulls down internal !global_shutter_trigger signal to lock image data onto per-pixel RAM on the sensor. 1/80ksec. later, the signal is released and intensity is recorded for retrieval.
These faster sync speeds are useful for situations when a) you want to use flash && b) the room/environment is too bright for a flash. Now you can keep ISO value to not way low, aperture kept open to F2.0, etc. without overexposing the image(can't do F0.3 at 1/80k; aperture is limited to up to F1.8 with some body recognized lenses attached, according to product pages)
When I was shooting professionally I certainly would have used the extreme speeds. Perhaps not down to 1/80000, but certainly higher than the "normal" flash sync speeds most cameras support today.
I used to photograph professional skateboarders. My favorite sort of shot would be of the rider doing something down a set of stairs using a telephoto (usually in the f4 and up focal length). I'd setup strobes in front of and behind the rider's line to get good rim lighting of their face and body as they nailed the trick.
That was to help isolate them in the shot against the background better, which, because of the f4 and need for a lot of light, we'd shoot in broad daylight.
That meant you needed very bright flashes and very fast shutter speeds. The best my camera could do at the time was 1/250 flash sync, which meant that was the fastest shutter speed I could use. 1/250 is the very bare minimum you can use in those conditions to freeze motion. If I could have shot above 1/500, that would have opened up so many other options for capturing those kinds of shots.
Others have explained that the flash duration itself doesn't need to be quick as the shutter. If it helps, think of it this way: you can still capture sunlight at 1/80000, even though the sun's "flash" duration is pretty long.
The only reason you need a flash duration that short is if you really want to freeze something that is moving very quickly. You don't need the shutter duration to be that short as well, unless you also want to exclude light that hasn't been produced by your flash.
For me, I've often wanted to shoot with flash at 1/500th or so outdoors, but most mirrorless cameras only go up to like 1/160th. Back in the DSLR days, there were a few camera bodies that would go up to 1/500th. Its insane to see flash sync beyond 1/1000.
Note that a full power strobe will burn about 1-1.5 ms, so below 1/1000 you start to loose the ability to balance flash versus ambient via shutter speed.
FWIW, you could do shutter speeds like that on the Canon PowerShot series of cameras all the way since ~2005 due to the unofficial CHDK - Canon Hackers Development Kit.
Weird artifacts and rotary motion are for all PRACTICAL purposes gone with fast, stacked shutters as in the Nikon Z9. The global shutter's main benefit is flash sync. Grain has nothing to do with global shutter....
Even with a fast readout, PWMing LEDs can still present issues for cameras like the Z9. The global shutter removes that concern entirely with no need for fine-tuning--still need some logic to identify the peak brightness but don't need to fine tune frame rates by fractions of a Hz to reduce banding.
A global shutter has been the holy grail for awhile for cameras like this in wide production. Yes, you can get a machine learning camera with 20 megapixels and a global shutter, but you'll need to strap a whole PC to it to do the data capture. And it'll be manual focus and manual aperture to boot.
Far as I can tell, OP isn't saying that grain has anything to do with global shutter, they're saying that rolling shutter produces artifacts, which unlike other photographic artifacts such as the often pleasant ones produced by film grain, are not at all aesthetically pleasing.
Unlike film grain, the weird artifacts for sports/action, camera moves, rotary motion... it's all bad looking and disappears completely with global shutter.