I don't see how it's different from paying for a physical copy which is pretty much a disc with a data on it. You can burn your digital copy to a disc and then it's basically the same thing. The only reasons to prefer a physical copy is if you want the box on your shelf and a manual to hold in your hands (or any other collector's item that comes with it), or if your connection's too slow to make digital purchases a viable option. Which are perfectly valid reasons, mind you. Also, Steam's pretty damn successful, so people don't mind paying for a bunch of ones and zeroes, apparently.
I know. Hence why I said this.