"Valve didn't set out to create our own prototype hardware just for the sake of going it alone - we wanted to accomplish some specific design goals that in the past others weren't yet tackling," Valve's Greg Coomer wrote on the Steam Community website.
"One of them was to combine high-end power with a living-room-friendly form factor. Another was to help us test living-room scenarios on a box that's as open as possible.
"So for our own first prototype Steam Machine (the one we're shipping to 300 Steam users), we've chosen to build something special. The prototype machine is a high-end, high-performance box, built out of off-the-shelf PC parts.
"It is also fully upgradable, allowing any user to swap out the GPU, hard drive, CPU, even the motherboard if you really want to. Apart from the custom enclosure, anyone can go and build exactly the same machine by shopping for components and assembling it themselves. And we expect that at least a few people will do just that. (We'll also share the source CAD files for our enclosure, in case people want to replicate it as well.)
"And to be clear, this design is not meant to serve the needs of all of the tens of millions of Steam users. It may, however, be the kind of machine that a significant percentage of Steam users would actually want to purchase - those who want plenty of performance in a high-end living room package.
"Many others would opt for machines that have been more carefully designed to cost less, or to be tiny, or super quiet, and there will be Steam Machines that fit those descriptions."