It prevents over / under fetching. Eg, on certain screen sizes you might show more or fewer fields. With GraphQL you can decide the fields you want in the client and explicitly request them. You can do this with REST by passing in a lot of extra query parameters, but it’s pretty awful. So most people end up either over fetching (getting more data from the server than they need and discarding some of it) or underfetching (get a list of IDs, then make a follow up request per ID). GraphQL solves that really nicely - the client requests what it wants, and the server answers it in a single response.

GraphQL is also typed, and has some really great developer tooling as a result. It’s usually really easy to learn about a data source by just exploring in graphiql with ctrl+space autocomplete. Eg, check out github’s API: https://developer.github.com/v4/explorer/