Use the Layers pattern when:

You want to decouple the data model, data access details, domain object mapping, or any other orthogonal features that you plan to alter independently.
You want to define multiple, incremental levels of software abstraction to simplify development and maintenance. You may also find that different abstraction levels are more amenable to supporting different types of client code. For example, you might write main application code in terms of the highest level of abstraction and graphical administration utilities using a lower level of abstraction.
You want to prototype or build a system gradually using stubbed or simplified layer implementations and then fill in more scalable or optimized implementations later in the development process.