Well in today's internet era, we want to make it as easy as possible for users to access content, in whatever way possible. In this case, we want to reduce the load times, and data consumption on the client side, but not have to refactor our API every time we do so.
This adds a certain type of flexibility about the application whereby the server team and client team agree on a
schema and the rest is dynamic. The front-end teams can outline exactly the type of data they need to remodel the UI without consulting the back-end teams. This development cycle will lead to faster turn around times and rapid prototyping, easily.
REST APIs are still valid, and useful pieces of technology, but GraphQL is how they can evolve to fit a more modernized, interconnected web.