A good design, following the principles of HCI, is the "make or break" of an application. With good design will come a good user experience. With poor design, the application will not be used to it's full potential.
A good design will earn the company a good reputation, and it will be easier on the developers. A bad design will cost the company more money because issues will take longer to find and fix.
Users are hard to predict, so a good design will account for all/most possibilities and therefore suit the users better and create more profit.
