Google Play System Updates mainly address security issues, but they aren’t the same as the monthly security patches. Both are responsible for different things. All devices with Android 10 and higher can get a Google Play Security Update, regardless of whether they have the latest security patches.
A good example of where Google Play System Updates could have helped was the Stagefright security bug in 2015. Stagefright was an attack on a multimedia player component in Android. The media framework is one of the 12 components that can be updated through Google Play System Updates. Many devices were never patched to protect against Stagefright because it required a firmware update.
The thing that bugs me about Android, and Windows, is that unlike Linux that runs an update and updates everything in one go, it seems that the Play System updates need to be checked and run, and with Windows it only checks after than application is run. Android does a similar thing where it seems to stop checking for updates to an app, if you have not run it recently.
We really need OS’s to check the operating system and all apps for needed updates.
The other thing with Play System updates is, they queue up if you have not updated, so you may need to run the check a 2nd or a 3rd time until it says there are no new updates.