In fact most of the developers do not manage the key themselves but instead use the default option: let Google generate and store one signed distribution key for you.

This is the key that is used by the devices to verify that the APK can be installed on top of another.

If an APK was signed by "Key A", only "Key A" can do updates.

One advantage of that, is that malicious users cannot be served a malicious update of an app that didn't go through Play Store.

Let's imagine it's the opposite, that France for example wants to spy on some users ?

Other advantage, is that if you lose your developer private key, then you do not lose the capacity of doing updates.

The minus of that, is that you have to count on Google to sign every single of your APKs.

If they do not trust Google, VLC can add a checksum or integrity checks to the binaries loaded by the app during runtime.