Nothing about GQL in particular is necessary to appreciate the need for this. Consider the Stripe API, where you might fetch invoices and expand the customers for each of those invoices, and then fetch the default payment methods for each of those customers. Stripe does this with a mess of query string parameters over a REST-ish API, but it could be accomplished much better with a request body.

GQL solves this problem in a particular way, but even folks who don't turn to GQL still have this problem.