Why would Valve want to throw money at a platform that is as close to locking out third party app stores as Apple is? Offering their client on Mac to those of

Why would Valve want to throw money at a platform that is as close to locking out third party app stores as Apple is? Offering their client on Mac to those of their producers and consumers who are inherently motivated to use it but otherwise keeping a low profile there seems like just the right call to me.