Functional contexts, such as design and architecture, have a job to do. If they fail to work properly and efficiently, then they are not made well. But as long as they function, people interacting with these objects will largely judge them by their appearances.
While designers (and others) may not fully know how to reconcile the needs for aesthetics, in light of the widely held belief that beauty is subjective, there are some exemplary (or paradigmatic) guidelines that have been effective. As a side note, the belief in the subjectivity of beauty is relatively recent (only a few hundreds years old). For a couple thousand years, people held a more objective account of beauty.
Immanuel Kant was wise to claim that there is no exact formula to guarantee that one will produce beauty. However, certain elements have been recurring in objects that have been thought to be beautiful. Proportion might be the most longstanding trait of beauty. While people may disagree about the exact which objects are beautiful (or to what degree), that fact does not preclude the possibility that there are some exemplary guidelines that could help someone make more effective design decisions.