Each app release is digitally signed with the same secret cryptographic key. When an Android device receives an app update, before updating it, the platform checks if the digital signature matches the version of the app that’s currently on the device. This process helps protect users by ensuring that only the original provider of an app can update it after it's installed.
Google offers a free service to developers that holds and manages app signing keys for apps distributed by Google Play.
By opting in to this service, you benefit from having the same protection that Google uses to protect its own apps' keys. If you're not opted in, and you ever lose your app signing key or have it become compromised, you lose the ability to update your app. By opting in to app signing by Google Play, you can also benefit from using Android App Bundles and Google Play’s Dynamic Delivery.