formality in case you need to reinstall

It's the pre-expansion patch that's adding all the new stuff to the game so you'll be ready to go come launch day. Assuming you're buying the physical copy all you'll need to do is type in your upgrade key on this website and then launch it. At this point the disks are just a formality in case you need to reinstall.
po world of Warcraft
You didn't honestly think all the new music, areas, cinematics, character models, weapon models and all the other stuff that I'm sure I've forgotten to mention was going to be some dinky little 500 MB update did you?

Tons of things to add in and I assume you don't want to have to download the full 14 GB come patch day (which I assume it two weeks from tomorrow)