People are moved to action by an emotional response to your product or message. Great design appeals to consumers emotion because it is beautiful, unique, solves a problem, functional, conveys an inspiring message and/or great story. Your product or service should make consumers feel something, and then act on those feelings. That decision can be to purchase a product, join a movement, volunteer, etc.
Every day consumers are inundated by thousands of products and messages. They have become adept at ignoring the noise. You have to give them a reason to listen to you by making them feel something when they see or hear about your product or service. Great design uses this emotion to convert consumers into sales and/or advocates of the brand or cause. It drives action and gives brands the power to influence the masses.