This feature was recently added to the Xbox One consoles themselves so it makes sense that this option would come to the mobile and tablet apps to provide feature consistency.
Here’s the official release notes for this update:
Get notified when your game clip or screenshot is uploaded to Xbox Live! We’ve added push notification options for game clips and screenshots, so you can watch and share your game media faster than ever, all from the Xbox mobile app.
This update also includes several bug fixes and performance updates to the app.
In addition to backing up recorded media of Xbox One video game gameplay, uploading clips and screencaps to Xbox Live also makes it easier to download the files on other devices and upload them to social media apps for sharing.