I see a '10 slot storage increase' discounted to 850 crowns, but doesn't go beyond regular bags. Why would anyone spend crowns instead of gold for that?
For new players early game, it can take a while to get the gold to max out storage. This offers a short-cut, much like buying skyshards on alternate characters. I personally wouldn't do either of those things, but I can see why someone with limited time to play and spare crowns might.