GrapheneOS is a locked-down, security-hardened system that's good if you need absolutely maximal security (e.g. journalists, activists, folks targeted by state actors). LineageOS is a more of an open system for tinkerers who want to play outside Google's walled garden.
You can have root to control your own device on Lineage, but not Graphene.
It's not really locked down. You can toggle or enable some of the more activist-orientated features. The only limitation I'm aware of is that some apps requiring the strongest Play Integrity setting (ChatGPT, some banks, very few airline apps) just won't work on GrapheneOS.