To improve code reusability and maintainability in Salesforce Commerce Cloud SFRA, defining a middleware function that performs common checks or operations and then applying this middleware across multiple routes is best practice. Middleware functions in SFRA allow for centralized logic that can be reused across different controllers or routes. This method enables developers to maintain a clean and organized codebase, where changes to the shared logic need to be made only once in the middleware function, rather than in each individual route. This approach ensures that the code is easier to test and manage.