Design conveys a message. It's another form of language that isn't spoken, but rather inferred. Whether you're trying to convince somebody to buy your book, or just presenting facts and figures to your clients on a chart, the design conveys your overall message. Personally, I'm especially fond of the clean, minimalistic designs that present a simple, direct, and focused message. It is worth noting again, that design is just another form of communication. Don't forget about the message it conveys!