It may seem like a minor or fair limitation, but the ability to release Windows-only titles is a key component in maintaining the dominance of Windows as a gaming platform. Letting developers release on Linux first might go a long way to increasing adoption.
I can imagine releasing on Linux a few months early, dealing with the issues not caught during development, then launching on Windows.
At the time it was called "Don't Be Patchman", apparently it changed its name and was ported to Windows and macOS.
https://store.steampowered.com/app/378410/Patchman_vs_Red_Ci...
personally i think we'd see much much much more console-ification of gaming, already, if Steam wasn't around, and to a lesser degree if Linux wasn't an option.
keeping game dev alive outside of the narrow walled gardens is very much a real challenge, a real problem, and Steam keeping a free & libre OS in a good state for gaming, & a flexible OS that they can be creative & take new risks & ventures on, unlike the fixed, closed operating systems, is more than a threat to keep the giant honest, it's a way to keep the ecosystem healthy, and a way to insure Valve can adapt & create & try new things. Linux, for Valve as for many others, is freedom, and it matters, here & now, & especially, ongoingly.
On a higher level note, this shows that Valve is pretty committed to Linux as a concept. I feel that while a second wave of Steam Machines or Steam OS may be a while away, they're certainly making it much easier for gamers to Switch to Linux if they were so inclined.
[0] https://www.youtube.com/watch?v=KrbWbBYAolo