Developers can also use an AMD card, but it requires more futzing and is limited to secondary "desktop" display mode. Intel graphics aren't yet supported, and Linux OpenVR game development requires the Unity version 5.6. There are a few other known issues: Base station power management and headset audio device switching aren't yet implemented, and as mentioned, you can't switch between direct (headset) and desktop display modes.
So why is Valve doing VR for what is clearly a very niche market? Valve's SteamOS is Linux-based, and it has hardware partners like Alienware and Maingear that sell Linux machines with the software pre-loaded. There's also a decent library of SteamOS and Linux games, so it makes sense to extend the platform to VR. The company is developing three new VR-specific games itself (using Unity), and it's very possible it will make those available on Linux (and possibly OSX) as well as Windows.
On top of that, Valve programmer Joe Ludwig recently said that developers are demanding SteamVR for multiple platforms and that the company is trying to limit "gatekeepers." That could be a reference to Windows 10 and its UWP apps, which developers worry Microsoft will use to cut off third-party markets like Steam.