Ex: you are building on top of third party ecommerce software.
You need to add more data to the Cart entity because you have a custom module/plugin/etc. You write a graphql resolver, extend the schema and the Cart type. You can use the existing built-in endpoint to query your new data.
So it's nice to be able to extend a framework and data access in that manner.
In general though, I do prefer REST APIs and then writing custom endpoints if I need data for a specific UI, like for visualizations.
I wrote some custom resolvers yesterday to extend graphql on a Cart entity and it works well for that.
I used some existing repositories the backend already had but was not yet available on the GraphQL side.
All the resolvers can lead to extra performance issues of course, but I suppose you can adjust your schema as needed and tune it with more efficient queries or code (ex: move some properties to nested and resolve with a single tuned query)