I dont use Graphene OS but my understanding was that it allows the user to sandbox Google play apps: https://grapheneos.org/usage#sandboxed-google-play
So this gives Google play less access than it normally would. I think thats probably the biggest draw of it other than security updates for longer periods of time for older hardware i imagine.
Again, I'm not a user of it so i cant say for sure but I've beem interested in trying it out as it seems up my alley as I'm getting more pivacy conscious as the years go by.
Thats pretty cool! My only gripe is I dont really wanna buy a pixel phone and I think there are unofficial roms for other android devices but idk if its worth the hassle.