Games being optimised for specific hardware.
Some games just run better on the PS when compared to the PC version, regardless of you having the latest and best PC. You can see this fairly well on the Nintendo Switch, which is a low spec tablet but the games run very well and the experience is great.
PC Games, generally speaking, tend to favor keyboard and mouse not controllers.
This is why I suspect game devs will start optimising for the Steam Machine, provided it sells well.
What do you mean by that? The PC experience with adequate hardware is almost universally better than the console experience.