Google announced this week that it will mandate the Android App Bundle over Android Packages for all new apps submitted starting August 2021. Google had introduced the Android App Bundle in 2018 as a new method of formatting apps for publication on the Play Store. It was adopted by developers as well as Adobe and Netflix.
Android App Bundles provided users with benefits in the form smaller downloads. An APK installation would need to work with all supported devices and languages. A bundle of AAB apps would only download the required app for your particular device and language combination. Google claims that they are more efficient for developers and result in fewer uninstalls by users. They also have greater security thanks to Google's Play App Signing which prevents apps from being compromised.
Developers used to have the option of choosing between APKs or AABs, but new apps that are submitted to the Play Store will not be able to do so. Existing apps are exempted, but the use of "currently" indicates that Google may change this requirement in future.
Developers will need to create completely different code bases to target each platform. This is because only Google can benefit from the changes. Developers are already facing difficulties because each app store uses its own APIs. This may make it more difficult for developers to support third-party apps stores.