Actually, it works a lot like gift cards. Stores sell gift cards to get money immediately, rather than later. When someone buys a gift card, though the store gets the same amount of revenue from the items bought for the value of the card, the store gets increased utility from having the money earlier. This means that they can invest the money however they choose, and most likely turn it into more money sooner.
The difference is that these EB and GS are more aggressive with their tactics, that instead of just offering a preorder, they press their employees to sell a certain amount of preorders among other things, because of the increased utility available from that preorder. I know I'm not the only one who's dealt with this type of treatment, I've even watched it happen to others, in multiple cities, as well. On top of that, I've read about it on the internet, from others, though this is less credible. Anyways, no I have not worked at a store like that, but a few years ago, I was one of those kids that were the great friends with all the other kids that worked in the store, and I have to say, these things were the bottom lines of what they told me. Other than that, I've read about the utility of Gift Cards and Preorders in economic reports and information.
I also know that the employees that work in an individual store are not the ones that actually order the items, there is either a manager of some sort, or a computer that uses an algorithm to determine how to order a product, that takes preorders into accounts.
Also, your comparison of gaming specialty stores to that of a department store is fallacious. You use size in your comparison, and in general, the size of a video game section in a department store is smaller than that of a gaming specialty store. Also, the specialty stores, at least every single one I've been in, have had stock rooms where they hold extra merchandise. Department stores typically do not store video game stock they have in their stock room, unless they receive the item before street date. They usually put all stock on the floor, and when they are out of any gaming item, if you ask a clerk about it, this is exactly what they will tell you.
Edit: So basically, if you care to be business-savvy, you know preorders are a poor decision, for loss of utility of your own money. Though, like I said before, there are a few cases where preordering secures you an actual copy of the game, whereas normally, you would have little to no chance of ever being able to buy the game new.