Graphic design is more than just aesthetics - it enables you to communicate with your audience. Companies use graphics in every stage of marketing to inform, delight, and eventually persuade the customers to purchase (or take the desired action).
Graphic design is a valuable tool if you want to inform your audience about an upcoming event or launch an email campaign to upsell a service to current customers.
Without great content, design is just decoration. But can you imagine a world without pictures?