There are many reasons why you might want to sideload apps on your Android phone, but there are also plenty of good reasons why developers would want to block sideloading. A sideloading app doesn't contribute to a developer's Play Store metrics, but it also prevents them from curating which devices can use their app. 
Improperly sideloaded apps may also not work due to missing components or code, or some features may be missing because you installed the wrong version for your device.
Whatever the reason, developers who want to prevent you from sideloading their apps now have a much easier way to do so with the Play Integrity API.
The Google Play Integrity API is an interface that helps developers "verify that interactions and server requests originate from a genuine app binary running on a genuine Android device".
It looks for clues as to whether the app has been hacked, whether the app is running in an “untrusted” software environment, that the device has Google Play Protect enabled, and more. If you've heard of or dealt with SafetyNet Attestation before on a rooted phone, then you're probably already familiar with Play Integrity, even if it doesn't have that name. Play Integrity is the successor to SafetyNet Attestation, only with even more features for developers.
As Google continues to strengthen its Play Integrity detection mechanisms and add new features, it will become increasingly difficult for power users to justify rooting Android.
At the same time, regular users will be better protected from potentially dangerous interactions. So very soon we will see Play Integrity start to be adopted by more and more applications.
