It's reduce the complexity of code and increase re-usability. it is flexible when we are working with team, so its possible to separate the task for designer, coder and tester.

By using theree tier architecture we can separate design, logic and database access. It is helpful for distributed environment and its easy to find bugs. It is easy to make changes in such projects where this architecture is used.

In three tier architecture the project is divided into three sections UI, BO and DAL. Due to this logical separation when we make changes in one section it does not ripple out throw out the project. Due to this projectmaintenance cost and complications are minimized.

UI: User Interface is a top layer in the application and it’s responsible for Look and feel and its run on client machine or user’s computer.

BO: Business Object resides in the middle below the User Interface and it’s responsible for validation
.
DAL: Data Access Layer is last layer or bottom layer of an application. All the database connectivity is done in this layer.

Below diagram show the concept of Three Tier Architecture in a better manner.
 

Please do also have a look at one moreimportant .NET question what are generics?

Please click here to see more  .Net interview questions