Twofold: for many, it just looks nice. But for me, having a little margin in between things helps me visually distinguish and makes it easier to look quickly a

Twofold: for many, it just looks nice. But for me, having a little margin in between things helps me visually distinguish and makes it easier to look quickly at different windows. This is actually helped with a wallpaper, because it further accentuates the borders of windows. I guess a magenta outline on every window would technically serve the same purpose (the sort of thing you might see on a high-contrast UI), but it's a little less gaudy and a little more visually appealing this way.