As computer builders and modders we spend hours on getting the inside aesthetics of our computers just right. We add things like water cooling and lighting along with whatever else we can think of to make our individual PC stand out from the crowd. In realizing this, the market has adapted offering windows in a majority of today's mainstream chassis, although like most things, there are always exceptions to the rule.
Perhaps you have decided on a case that has exactly the features that you need, but has one downfall - it hasn't got a window. A prime example of this is the BitFenix Prodigy or Cooler Master Cosmo II. Other examples where a window mod may be handy are cases like the Antec DF-85 or Cooler Master HAF-X.
Both are factory installed with a window in the side panel, but these cases have uniquely designed windows that don't really take full advantage of the size of the panels, and instead concentrate on airflow. However, maybe airflow isn't your thing, and you would rather add a huge window showcasing your PC's glorious interior.
Join me as we venture into the world of window modding!