GraphQL schema delegation allows for the automatic forwarding of GraphQL queries or parts of them to another schema without manually querying them. This enables reusing parts of other schemas without having to query them directly. Schema delegation is particularly useful in scenarios such as building a GraphQL gateway that forwards queries to underlying microservices, using a third-party GraphQL API, or utilizing a GraphQL database proxy like Prisma. By implementing a `Node` interface on top of the service schema and reusing existing types, the gateway can delegate root fields from the original schema to the delegated schema, allowing for more efficient data retrieval and reduced overhead. The `delegateToSchema` function in `graphql-tools` simplifies this process by automatically filtering out incompatible fields and fragments, as well as unused ones, making it easier to implement schema delegation. Additionally, schema delegation can be combined with schema stitching to create a gateway schema that combines multiple services, setting up automatic delegation for root fields that already exist in the stitched schemas.