“All of this work is broadly applicable to the PC platform, and it’s going to continue to expand over time. Supporting multiple platforms, multiple chipsets, controllers for different machines that are out there and even ones that aren’t out yet.”
[…] Valve’s goal with the OS is to have it compatible with traditional PCs, laptops, portable consoles and any other formats.
It’s about having a plan B incase MS starts locking in the customers into their own gaming services.
Tim Sweeney warned about this when they tried to push the Windows Store on Windows 10.