This!

But I'll add one more: "Standardisation". Every RESTful API uses different URL paths, naming schemes, HTTP verbs, data structures, headers. Whereas those are defined by the GraphQL spec.

When consuming or creating a GraphQL API, those (usually meaningless) decisions are defined by the spec, so I only need to learn how the data is structured. Since it's a spec, my queries can be strongly typed in both frontend and backend.