It lets the client decide
If client coders just want a small subset of the data, then traditionally they have to ask the developer of the server-side REST API to provide a method that returns just this subset. If there's only a standard GET method that returns everything, then the client has to incur the expense of calling that and then discard most of the response.
By giving the client the ability to select just what they want from a standard endpoint, GraphQL has the potential to speed up client/server development and/or interaction. However, there's the risk that in practice this approach will just transfer complexity from the API to the client.