For communication
We design mainly to convey a message. That message can be that the viewer has to appreciate it internally and/or to make them take external actions.
Let's take an example of when you meet a banner in town with a preaching message and to it is added the verse John 14:14.
You would be pushed to find out what that verse says if you did not know. The creator of that banner provoked a movement in you. Then after reading the verse, you start to think about the saying. In the end, you will judge it either as good for you to take or bad for you to take. But, one thing is sure you will make an internal decision. Plus, that can remain in you for so long. That's why design is called visual communication. And it's one of the most powerful communication media.