Every company I've worked for in the last 10 years had multiple apps across several platforms, each with many versions, all running in production at once. Even if you only care about the next version you are shipping and a handful in production that's a dozen variations.

In a REST paradigm you over fetch because not all of these variations need the same data, or you send less but then the clients thicker because they merge API calls and have divergent presentation logic, or you have a bajillion API versions.

There's often a lot of back and forth between the various teams for each rev of the API.

GraphQL lets the clients drive the definition of the data fetching. That's it.

95% of the criticism of GraphQL is people complaining that GraphQL doesn't solve the problem of preparing the API response for these different requests. While that's true, that's not what it's supposed to provide. Whether you have a REST API with 26 versions or a GraphQL API with 26 variations of query you're going to have to write a backend-for-frontend style service that resolves the results.

GraphQL just standardizes this process.