In 86 slides, Stephen Anderson explains why it's impossible to separate visual design from the core of the product. You can't just make it pretty later. Visual design has to be baked in from the beginning.
Visual design and understanding of gestalt give you the tools to realize your intentions. Interaction design is knowing what a user should be able to do on a given step. Visual design is making something that allows the user to do it, quickly, efficiently, easily, and all the while feeling good about themselves.
In consumer products, how the customer feels when using your software is make-or-break.