What data do I actually want to carry across websites? I actually don't understand the value here. Is it that I don't have to re-enter my personal information

What data do I actually want to carry across websites? I actually don't understand the value here. Is it that I don't have to re-enter my personal information on each site? Signing in with Google already gets me that. Is it something else? Is it some kind of JSON blob that every service will know how to ingest properly?