Simplicity is one of the most powerful techniques you can use when marketing or advertising. Things that are simple are also easy, and when you're trying to inspire someone into an action, easy is good.
Consider a Web site. While your natural instincts may tell you to add a lot of elements and gadgets, this can often lead to confusion for the user. Since Web sites are now one of the main avenues consumers use to find product information, find contact information, and make a purchase, it's absolutely necessary to make sure that your Web site is easy to navigate and understand.
Logos provide yet another example of the positive power of simplicity. Logos are created to stick in the minds of consumers. The more complex, the more there is to memorize. This makes it much more likely that your logo will not be remembered.
In short, simplicity in marketing or advertising increases the chances that your company will be considered by a customer because it eliminates potential speed bumps on the road to a purchase.