Answer:

This is a very popular c# interview question. In 3 tier architecture / layer we divide the project in to 3 layers UI , Middle  and DAL.  Due the modular approach we have 2 big advantages

Reusability :- You can reuse the middle layer with different user interfaces like ASP.NET , windows etc. You can also reuse you DAL with different projects.

Maintainability :-  When we change in one layer due to the modular approach it does not have ripple effect on other layers. we have to do less amount of changes in other layer when we change logic of one layer.

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