A Headless CMS means you define all of the frontend interpretation of the content. Basically a GUI for your database.
If you have lots of queries happening behind the scenes whenever someone makes a request that seems basic- like how Facebook gets a list of comments on a post, plus the friends who made them, and a list of different reactions to each comment, and other stuff that rapidly creates a complex graph- then GraphQL simplifies. The REST way involves all of those extra calls, GraphQL gets everything in one request.
If you have lots of queries happening behind the scenes whenever someone makes a request that seems basic- like how Facebook gets a list of comments on a post, plus the friends who made them, and a list of different reactions to each comment, and other stuff that rapidly creates a complex graph- then GraphQL simplifies. The REST way involves all of those extra calls, GraphQL gets everything in one request.