The responsibilities of each layer

• Controller - bridge between the model and view. Decides where to go next.
• View - displays the data, gathers user input
• Model - business logic, interface to data store.

One of the biggest gains is in maintenance and expansion

• If you need to change business logic, you should not need to modify your controller or view.
• If you change your visual display, you should not need to modify your model or controller.
• If you change your workflow, you should not need to modify your view or model.

Source