Stage variables allow developers to adjust configurations for different deployment stages without altering the core definition or duplicating the resource. They provide a way to alter the behavior or endpoints that the resource interacts with depending on the stage, whether it's for development, testing, or production. This avoids the complexity and redundancy of managing separate instances for each environment or using other services that are not intended to perform this role directly within the context of serving APIs. Unlike stage variables, separate instances and Lambda function deployments add unnecessary complexity and potential cost. AppConfig, while useful for application configuration, isn't a direct substitute for stage-specific variables in the context of serving APIs.