Extension methods are very powerful; they can help you move code outside of automatically generated code, so you don't have to do any extra maintenance work there.  They also can attach methods to objects that are sealed or not-inheritable in VB.NET. But one of the more important facts is that they can reduce the amount of code and create reusable logic that is simpler to execute.

Extension methods, used wisely, can be a very powerful thing, and can add a huge convenience for an application.  It also encapsulates code and can assist with code reuse by placing it in the static class where it can be reused for every instance of the extended type.