So you're buying a case and a hard drive, you have to put it together yourself, and it only works with a raspberry pi 2? Seems like anyone interested enough to buy this would just do it themselves.
A normal external hard drive risks not working with a Pi. This 2.5" USB drive is optimized for low power usage, esp at startup and comes with a cable and charger which powers both the drive and Pi from a single cable.
Obviously, if you are looking for an external USB drive for your computer, this is all not relevant. But if you want to use it with a Pi or a similar board, this kit is far superior.
However, if you look at the bottom it states that this case is a reference design, much like the Google phones, intended to be a platform upon which others can build new products.