I think a big part of that, for me at least, is Steam Input plus the trackpads being an actually usable alternative to using a mouse to move a cursor.
Those two features have made countless games that I never imagined I’d be playing on a controller, let alone a portable device, completely playable.
Steam Input alone is such a killer feature that I rarely see people talking about. The amount of customization you can do on a game-by-game basis is actually kind of hard to believe.