That isn't to say that compatibility with it won't be integrated into more apps in the future. Rather, Google is starting small, probably with apps it feels are a priority. As well as being able to best showcase Motion Sense's strongest points.
YouTube will be one of the launch apps for Motion Sense
One of the launch apps for Motion Sense will perhaps unsurprisingly be YouTube. This shouldn't come as a shock. One of the advertised functions that Motion Sense provides is waiving your hand in front of the phone to manage media playback.
In that regard, YouTube fits quite perfectly, as you can use gestures to pause, play, and skip ahead or rewind in videos. This will presumably be quicker than interacting with these functions on the display directly.
In addition to YouTube, a couple of Google's other applications will support the feature too. These include YouTube Music, and Google Play Music.
The remaining apps all pertain to media too
Every remaining application that works with Motion Sense revolves around media. Again, this is likely because of how Motion Sense works and media applications mesh well with it.
The remaining applications which will support Motion Sense at launch will be Spotify, Deezer, Amazon Music, iHeart Radio, Pandora, and Spotify Stations.
That isn't a very huge list. But, even if Motion Sense really ever only works with media applications, there are many more to fill out the Pixel 4's compatibility list for it. Chances are it will come to the likes of Netflix, Hulu, Disney+, HBO MAX and any other application that lets you play media in any capacity.
Keeping that in mind, no other applications have been confirmed just yet. On top of that, there's no telling how long it will take for the support to be added to additional applications. It's also possible that Google may have other apps in mind for support.
It may even announce those apps at its upcoming October 15 event. Even if they won't be ready for use with Motion Sense by the time the Pixel 4 is available.
Beyond controlling the media inside specific apps, Motion Sense will be capable of doing other things too. It'll be the power behind the new Face Unlock feature on the Pixel 4, and it could be expanded to work for other functions in the future.