With Mac OS X, Apple introduced the new Aqua interface. Then Metal. Then the iTunes metal look.

They're all nice, but they also all offer slightly different visual cues, and different draggable areas, and different clickable areas. I find this ever so slightly annoying.

Uno fixes Apple's fragmented interface. With Uno, you get one look, the iTunes Unified look, across the whole operating system. This creates not only visual consistency, but behavioral consistency, too. The Unified look is not my favorite, but having one consistent interface, everywhere, makes up for the minor visual deficiencies.