The objective of layered architecture is to provide separation between each layer. Each layer should be designed in such a way that it has very little, or no impact on the other layers. There are different techniques to achieve decoupling between layers and interfaces are one of the techniques that are primarily used. Instead of using the concrete implementers, we use interfaces to communicate with the other layers and the changes do not impact on others. For example, there are cases where we may want to change the database engine in the DAL and instead of using SQL Server we can choose Oracle. Or, in the case of the UI, we may want to change the Web Forms framework for an MVC framework. Good architecture always provide this decoupling and reduces dependencies.