Don't think that there is a purpose. It's just simply because we can, I think!
Some people like the look. Some people like black wallpaper, some like many colors. Some want transparencies on everything, some wanna see how much crap they can cram on their screen.
I think it just comes down to, everyone being different and we all have different ideas of what is aesthetically pleasing.
I like the clock toggling, simply for the option, to display one or the other. Not so much both.
Short answer = no point really, don't need one!
