Is it really a distinct benefit though since something similar to that have been existing for a really long time before GraphQL already? From OpenAPI/Swagger all the way back to HATEOS/HAL and RDF, if not even earlier.

I'm not generally a fan of graphql, but yes it's a remarkable feature. The other options (like swagger) take more effort and tend to not be maintained well. But even when they are, it's still not as developer friendly (to the client) as graphql.

I've worked with all of those options before I started working with GraphQL. The "out-of-the-box" support, especially the standardization that GraphQL enforces with its schema definition, is/was night-and-day compared to something like Swagger.

It is a distinct benefit when your query playground is schema-aware and can suggest completions for types, fields and field (query) parameters, as well as validate everything pre-request.