Clients can use it to request exactly what they need. Since it's a graph, you can just keep grabbing nodes to get the fields you need in the shape that works for your client. GitHub added a GraphQL endpoint for their API, the improvements they cited were reduced overfetching of data the client didn't use e.g. urls to linked resources, and fewer underfetches e.g. using the results of the first request to populate a second or third request.

http://githubengineering.com/the-github-graphql-api/

It's not really GraphQL vs SQL, it's GraphQL vs RESTful APIs.