GraphQL helps where your client needs a flexible response format to avoid extra queries and/or massive data transformation with the overhead of keeping them in sync.
Using a GraphQL server makes it very easy for a client side developer to change the response format without any change on the backend.
With GraphQL, you can describe the required data in a more natural way. It can speed up development, because in application structures like top-down rendering in React, the required data is more similar to your component structure.