Good design is not just about looks, it is equal parts form and function. For instance, no matter how nice a chair may look, you would not say that chair is well designed if it crumbles under the weight of a person. The same can be said of communication design.

What good is a postcard if the primary message is never read by the viewer?

A well-trained designer knows how to intentionally use the principles of design to make things both beautiful and functional. Design is not just dumping content on a page, there is intention behind the placement of every element.