Sorry to sound dumb, but what is SDK, and what does it do that the normal 5D3 doesn't?
SDK is Software Development Kit
Basically, it allows software developers to do things to the camera while it's plugged into a PC via usb.
In short, you don't need it.
But software like Lightroom and Capture One, to use the the 5D3 in tethered-mode (like you can do with Canon software already, whatever it's called), or for the Reikan FoCal auto-AFMA software to do its thing.
So you may not need it, but you're more likely to use some software that couldn't be made without it.
Still, once the SDK is released it could be anything from a week or two of testing to a few months of full software development before we see anything based on it...