Probably for one of the reasons graphql was created in the first place - accomplish a set of fairly complex operations using one rather than a multitude of API calls. The set can be "everything" or it can be "this well-defined subset".
I think they mean something like (or what I think of as) “RPC calls, but with the flexibility to select a granular subset of the result based on one or more schemas”. This is how I’ve used graphql in the past at least.