Good design represents a visual “language” of sorts: when deciding upon the elements of your interface, your book, your map, you are designing the language that you are using to communicate with your reader or user. You are, in other words, giving the subtle cues which say “stop here”, “this is important”, or “this is what you need to do next.” Subtle cues can also indicate things like “I’m doing something in the background” (the rotating network busy icon in the top of the iPhone) or “You may not be able to do that here” (the status bars indicating signal strength).
Design can also save a life. Literally.
A friend of mine was telling me a story of a middleware company that almost killed someone. Their software is used in the medical profession; one element of the software would print the paperwork a pharmacy uses to fill a prescription. And the problem was that one elderly gentleman got a bottle not met for him because in the dozen or so bottles to be filled, the workers at the pharmacy accidently added someone else’s prescription to the stack of pills intended for that elderly fellow.
While he blamed it on user error, the reality is that this is a failure in good design: all it would have taken was a cover sheet between prescriptions to serve as markers between prescriptions intended for a single recipient. Or (since they provided the hardware) a specialized printer which can collate and staple automatically. Or even redesigning the page so that multiple prescriptions are fit on a single page as much as possible, with a large (and easy to read) page number and total number of pages in one corner, along with a count indicating the total number of pills to fill for this individual.
Any of these visual cues would have made it easier for a worker to notice something was wrong: as soon as the prescription count resets to 1 or the page count resets to 1 or he hits a cover page, he’d know that maybe he should read the patients name a little more carefully for this batch of pages, rather than shove potential lethal poison at an elderly individual who doesn’t know better.