<< OK, maybe I am just dense, but why would you want a window in your tower? >>
Why would someone want his optical drives to color-match the rest of your case? Why would someone not want his case to be puke brown with pink dots and jungle green stripes?
It is because people want their cases to look good to varying degrees. Some people want something that's not so hideous that it's distracting, some people want something so flashy that it IS distracting, and others want something in the varying shades of grey.
As for windows specifically, it's very cool to be able to see the inside of a computer while it's running. Just like some people marvel at the inside of Big Ben (you know, that clock in England) or are fascinated by a running steam engine locomotive, some people like to see the inside of a computer while it's running. Sure, there's only a couple of LED's and a few fans spinning, but you get to see it. You know empirically that the inside of your computer doesn't turn into a hamster running on a wheel, and you know it all the time. For the home builders, this is particularly important, they get to see their hard work. Not just the output on the monitor or speakers, but the physical device they put together. They get to see it as much as they want, when they want, simply by turning their heads.
Also, if some 400g heatsink falls off your CPU and makes a Godzilla-esque path of destruction throughout the inside of your case there's a good chance you could see it happen with a window. Isn't that alone worth the extra bucks?