Well, let me ask you a question: Have you changed the wallpaper of your pc, or are you perhaps still looking at some generic green rolling hills?
Personally, I spend virtually no time without maximized applications taking up all my screen real estate, so the actual wallpaper gets next to no screen time. Despite this, something in my nature compels me to, perhaps irrationally, immediately change the wallpaper and theme on any new hardware I acquire.
For me, it's actually not about subjective color and graphical preferences as much as it is about a sense of ownership and personalization. It's just something that makes your piece of hardware feel more familiar and comfortable, an electronic "home away from home" if you will.