N-tier application architecture provides a model for developers to create a flexible and reusable application. By breaking up an application into tiers, developers only have to modify or add a specific layer, rather than have to rewrite the entire application over.
In SharePoint this makes sense no? I have seen MOSS and Foundation projects where everything it’s mixed so all point’s of failure are in the same place and it’s more confuse, to not say a mess, understanding the business logic, N-Tier Architectures make application more robust because there is no single point of failure. Tiers function with relative independence, and this in SharePoint makes the difference.
N-Tier Architecture helps developers build web applications because it allows developers to apply their specific skill to that part of the program that best suits their skill set. Graphic artists can focus on the presentation tier, while developers can focus on the code tier.
Let’s see this example diagram, this is only a very simple example
The important concept to note here is that if the interfaces between an n-tiered application's layers remain constant, the implementation of each layer can be easily upgraded or changed. This is also a fundamental concept of good object oriented design. Another advantage is that you can reuse some parts of this project, like the Claims Signin Page.
There are almost an infinite number of architectural variations you can use when developing SharePoint applications. The best ones are both easy to create and maintain