Neat trick but why would anyone want to use something like this?
It helps to build a community around Digikey in preference to other vendors, giving them greater mind share?
Simple free schematic drawing packages are not easy to find (I know, I've looked), however ordinary the concept might seem.
It's a nice way to get an all in one solution, perhaps? You can sketch out your design with components you know Digikey will have in stock, so that when you are done you can just get a BOM and place an order. Easy! Much preferable to separately searching for components by availability and possibly having to split your order between more than one vendor.
OK, for the pro's this is no big deal. But for small or one-off projects the convenience could easily make using such a tool attractive. I think it would for me.