What is GraphQL?: what you defined is what you will get.
GraphQL offers a solution to the issues of under-fetching and over-fetching data present in traditional RESTful APIs by allowing clients to specify the exact data they need through a declarative language
When you may want to use it:
data source complexity: apply graphql server as the middleware to glue multiple data source, either during BE refactoring or FE needs to work with multiple data source
Project with anticipation of future complexity on data
What is it and When to use it