I never understood preordering of a digital copy. You at least in most cases get the physical copy in your mailbox on the release date. But with a digital copy you could download it right away on the release date.
I think it might just be a matter of just being able to go ahead and make the purchase ahead of time so you don't need to worry about it later on. I know of plenty of people that prefer to purchase things like that well in advance just so that they don't have to scramble for it at release date. Besides, with all the other thousands of people going to purchase it at the same time (if they do it at release) who knows what congestion issues there might be as far as getting your purchases to go through.