there are a bunch of upsides. for example, querying with graphql allows the client to be very concise in what they want (no over/under fetching) and to traverse relations in a single request

there is a benefit at decoupling between client and server

this is one of the things i like most about graphql, that you can fire off an infinite amount of queries to the backend without the backend having to change anything. compare that to oldskool REST