Translucency adds more dimension to your desktop.
By adding translucency to certain interface elements in OS X Yosemite, we’ve put a greater emphasis on your content. Translucent toolbars let you know there’s more to see than what’s visible in the window as you scroll. And a translucent sidebar lets you see what’s hidden behind the active window. So the interface takes on the look of your desktop image and your content — making your Mac experience different from anyone else’s.