It's lower friction in that instead of having to click through an OAuth screen (and possibly re-enter your Google password), you just auth with FaceID/TouchID. And if you're writing an iOS app, then obviously the user has an Apple account.
I'm on Android, so I don't know how it works on iOS devices, but is reentering the Google credentials common? On my Android phone I doubt I had to enter my password more than 5 times in 2 years.
And the OAuth screen would be required anyway, as they have also shown on stage.
I frequently have to re-enter my Google credentials on the web on Google properties. I assume Android is storing those deeper in the OS, like Apple is storing your Apple credentials.
Some apps don't open the "sign in with Google"(/facebook) popup correctly and so cookies end up not being enabled for that browser session, so users need to re-enter their google credentials.
This is being fixed over time with proper implementation.
