Short version: Digital market does not have limited stock, therefore preorders are pointless - you get the same amount of the same product, regardless of whether you're purchasing it before or on release, so preorders are of no benefit for a customer. However, preorders are very good for publishers - they can get money for a product before it even gets released, before there are any reviews or opinions on a game out there - basically, they can get some money for a product which is an absolute turd. That's why publishers manufacture the last issue - preorder bonuses. And yes, even CD Project is guilty of this.
Long story short: To get money for a product which customers can't find any reliable information about, publishers bribe customers with cheap trinkets which will then never be availible after release, even for people who paid full price. 100% anti-consumer, 100% pro-publisher.
And yes, you can argue that CD Project is good, honourable and their games are great or whatnot, but that doesn't make the practice itself good - it's the same thing as if CD Project started putting DRM on their games, but you'd say it's good because it's by CD Project.
Long long version by TotalBiscuit, actually explained down to fine details