https://wccftech.com/google-stadia-app-got-downloaded-over-175k-times-in-the-first-two-weeks/

The Google Stadia app, available on both Android and iOS, may offer the best insight yet to approximate the userbase of the newly launched cloud-only game streaming platform.

That's because the Google Stadia app is required to purchase games; for some reason, that cannot be done via your PC for the time being. Of course, the app itself can be downloaded without having an actual Stadia account, though that'd be pointless on its own.