The main benefit I see is the ability to define a mutable model schema that is backwards compatible with the ability to perform complex queries and output them in a single request.
Instead of defining different queries and endpoints you define the schema and mutators based on object types. Then when you ask for a complex list of objects they mutate depending on the models schema functions on a per type basis... i.e. GraphQL will look at what was requested, find the data's requirements, and shape then return the request data.
"GraphQL server operates on a single URL/endpoint, usually /graphql, and all GraphQL requests for a given service should be directed at this endpoint."
"Typical REST APIs require loading from multiple URLs, GraphQL APIs get all the data your app needs in a single request. Apps using GraphQL can be quick even on slow mobile network connections."