Aesthetics are important to humans. We all love nice things. They are representative of our values. Values are beautiful because they define our actions through life.
Different humans have different ideas on what's beautiful and what's not. They are based on culture, surroundings, education... Something that's beautiful to a truck driver from Sweden might not be so appealing to a mathematician from Tokyo.
Beauty is in the eye of the beholder.
The main objective of design is to convey the message to the audience, and to evoke an emotion.
Design uses visual tools to make stuff easier to understand, to remove friction between the sender and the receiver, to get the message accross as clear as possible.
We are not our target audience. That's why we spend a lot of time understanding the real audience, talking to them. We'll put ourselves in their shoes.
We'll sketch an idea and show it to a sample group representative of the wider audience. We'll observe how they use it and ask questions. We'll then iterate, and test again.
A number of principles and techniques emerged over time that help deliver messages more effectively. Gestalt principles, color theory, compositional techniques, motion, storytelling. We study them and use as tools to get the job done.
Design is solving problems. That's what we do.