Ok to word it better. There is no availability on PC on the companion app, only on mobile. There is a remote play option to control your console from other devices witch in theory should be a great substitute. The problem is it next to never working for anyone who aren't on the same network as their console.
To reply to your statement of "it would make no sense to do it" that's not how PC ports of the Xbox work on the app. They work on the Xbox servers allowing you to play your Xbox games on PC( sometimes mobile). The point is for us Xbox players can have another good option when we're not at our Xbox this making it portable without using another client such as stram or starting over.
Good examples of this is destiny 2 and pso2 for those who don't care for cross play but have good laptops to play on the go when they get a bit of time away from the console.
Besides I don't have to bring up the difference between potentially streaming a game versus launching a game.