All Design leads to usability of the product, application or website. The primary goal of UX is to deliver functionality of a product in a way that is intuitive to use, responds well to the user with a focus on safety. Any design that is ascetically pleasing does not automatically deliver a good usable product. Making usability the foundation of design is essential in ensuring a better CTR (click through rate).
Usability is the reason why prominent buttons and a minimalist design with few elements increase the click-through rate (CTR). Precision Marketing Group doubled the CTR on their call-to-action button by changing the shape and size of the button.
Usability testing asks users to interact with the design. Meanwhile, you take notes to see whether they encounter any problems.
If many people experience similar problems, you will have to make changes to the design to sort out the usability issue. Keep in mind that design is an iterative process and it demands improvements in all stages.