Controllers are a vital part of any MVC web application. A controller is responsible for handling incoming requests and producing an appropriate response. The controller is where application logic is implemented. It is where the application's business logic is realized. This includes tasks like validating user data, updating models, and manipulating data to produce the desired output. A controller is responsible for determining which view will be used to display a response, as well as what data that view should have. Essentially, a controller ties together models, views, and other components to create a cohesive web application.