A good UI designer makes buttons look like buttons so our brain can quickly recognize them as such, without mistaking them for something else. A good designer has the window title in the same place no matter the application, so we know where to look before we even look. A good designer gives us a place to reliably perform common actions such as dragging the window.
We use our computers daily, and even imperceptible improvements to cognitive load and the amount of time it takes to perform actions make a difference over time. When I use my 2009 iMac running Snow Leaoprd, I perceive a reduction in friction compared to the latest macOS versions, and even compared to Catalina, which I use as my primary OS.