Middleware, in the context of web development, refers to a series of functions that are executed in sequence during the processing of an incoming request. KeystoneJS employs middleware to intercept and manipulate requests before they reach the route handlers. This provides developers with a powerful mechanism to add custom functionality, authentication, validation, and more. 

• Customization and Extension: KeystoneJS middleware allows you to extend the framework's core functionalities according to your project's unique requirements. Whether you need to preprocess data, implement custom authentication, or integrate third-party APIs, middleware provides a structured way to achieve it. 
• Error Handling: Middleware is an effective tool for managing errors in a centralized manner. You can create error-handling middleware to catch and handle exceptions across your application, ensuring a consistent and user-friendly experience. 
• Data Transformation: With KeystoneJS middleware, you can modify request and response data, transforming it to meet specific needs. This is particularly useful for tasks such as data validation, formatting, and localization. 
• Security: Implementing security measures is crucial for any web application. KeystoneJS middleware enables you to enforce security checks at various levels, safeguarding your application against threats and vulnerabilities.