The people who write the API would need to know you want that data and implement the change, or write a new endpoint that returns the user details and the friends details in one request. With GraphQL the API wouldn't need to change; you'd just need to write the query on the frontend and it'd work.

For most web apps the overhead of adding a new endpoint is small so the benefit of GraphQL is also small, but if you're Facebook then GraphQL makes a huge amount of sense.