If you have already uploaded APKs to Google Play, whether manually, via the
"Upload Android APK to Google Play" post-build action, or by other means, this
build step allows you to change the release track for specific APKs.
For example, you can use this to promote an app currently in alpha testing to
the beta release track, once you've decided it's ready for a wider audience.
Similarly, once you're ready for release, you can move from beta to a staged
rollout, or directly to production.
Note that "downgrading" release tracks, e.g. from production to alpha is not
possible.