A real world implementation:

You can cast an object as the Interface type:

IHelper h = (IHelper)o;
h.HelperMethod();

You can create a list of an interface

List<IHelper> HelperList = new List<IHelper>();

With these objects you can access any of the interface methods or properties. In this manner you can define an interface for your part of a program. And build the logic around it. Then someone else can implement your interface in their Business objects. If the BO's change they can change the logic for the interface components and not require a change to the logic for your piece.