Valve probably ported their stuff to Linux as a hedge against the Windows and Mac App Stores. It barely even matters if people use it or not; they just need a way to convince Microsoft and Apple that breaking Steam isn't in their best interests.
Could also be a niche revenue stream in future if they starting build it now: A few old games (and modern ones) run better on Linux via Valve's work than they do windows, that mean's Valve can still sell them.