TL;DR: Microsoft's upcoming Xbox Project Helix will support both Xbox and PC games, making the Xbox app on PC's introduction of third-party game support for easier access a sign of things to come.
In the name of convenience, many apps these days are designed to be cross-platform. They can run on both mobile and desktop, so you can readily switch between devices without starting from scratch. It ...
This lets you add games and apps to the app from any source.