A proper REST API would also work without all the extra overhead of GraphQL.

People may dislike XML, but it is easy to make a REST API with and it works well as an interface between computer systems where a human doesn't have to see the syntax.

Depends mostly on efficiency: GraphQL (or Odata as a REST compliant alternative that has more or less the same functionality) provide the client with more controls out of the box to tune the response it needs. It can control the depth of the associated objects it needs, filter what it doesn't need, etc. This can make a lot of difference for the performance of a client. I actually like Odata more than GraphQL for this purpose, as it is REST compliant, and has standardized more of the protocol.