Design Pattern, How Important is it

Design pattern is usually be used in software application programming. There are some design pattern which is used widely by enterprise, or insividual programmer. But how important is this design pattern 'thing'?

The main purpose of design pattern is to separate the application interface (UI) with the business logic. Why is it needed to do such thing?

In my latest job, there was a project which need to be handed over to me. The project are using Asp.Net webform. The structure of the project are using event-driven structure, as the basis of Asp.Net webform design.

The business logic (lets say that as the logic to submit a request, validate the form or updating the request) are being done in code behind of aspx.cs form. To be worse, the business logic sometimes handled in asmx webservice and being triggered by jquery ajax, making it harder for me to decrypt it.

Well, the pain did not stop there. The design are making it harder to be modified. A little modification can cause errors in other places, and more effort are needed to unify the change in other places as well. This is, are contrary with principal of object oriented, which is encapsulation and reuseability.

So how can a design pattern be used to solve these usually founded problems? I will try to describe it in my future posts.