It backs up... Photos, basically? With an iPhone you can take a complete image of the device, and never lose a file again. With Android, solid chance you'll never migrate to a new phone and not lose stuff. It's such a fucking shitty thing that doesn't need to be.
Honestly, I can't think of a single app that doesn't back up its own data. Meta apps have their own backup mechanism, games all backup either through their own servers or Play Games service, pretty much everything I use is a service of some sort.
But yeah, I guess Google isn't backing up the state of the offline chess app I downloaded 10 years ago...
I've gone through like 5 Android phones back to back. Photos, documents, emails, installed apps, contacts... It's all seamless and works, literally just with Google's defaults.
What do you think you lose?