PixelImpact The main advantage of Google App signing is that the main key is held safely by Google, meaning you cannot lose it or have it stolen and compromised by a malicious 3rd party.
Normally when you sign an application you are basically saying that you made it. Whatever system runs your application can see if it's been tampered with, and decide not to run it as well. Unless someone has your key and passwords they cannot sign as if it was you.
In the Android ecosystem you sign the APK and it goes unmodified until it reaches the user, so the users device can tell you signed it. If the key changes then neither the store or the user can confirm that you created the APK, so it's considered invalid.
The Google App signing changes this. You sign the APK, and the store verifies that it was you that signed it. Then the store signs it with a different key it holds securely. The end user only ever sees the last key. This means if you need to use a new key, you can securely tell Google your using a new one and to not trust the older one anymore. Users don't see any difference.
There is a second reason to do this. If you decide to use the newer Android App Bundle (AAB) format you need to use app signing with it. The reason being that AAB files are like a half built APK. The store decides what parts of the AAB are needed by the user, then creates an APK from those parts and signs it with the key.
I don't believe you can swap to using Google App Signing once you've published an app, because the users device would not recognise the signature. But I haven't read into it much.