Designs that are aesthetically pleasing are perceived as more usable by users.
Definition
The Aesthetic-Usability Effect refers to users' tendency to perceive attractive products as more usable. Even if two designs have identical functionality, the one that is more visually appealing will be perceived as easier to use.
Do
Invest time in making the design visually appealing
Use aesthetically pleasing color combinations
Ensure the design is visually balanced and harmonious
Avoid
Neglect the visual aspect of your design
Assume that functionality alone will make a design usable
Underestimate the impact of aesthetics on user perception
When to apply
Designing the user interface for a product or service