Windows are there to provide light and air into a room, but what happens if you want neither? Obviously, most windows are made in such a way that their framed panes can be shut to make an airtight seal these days, but that won't prevent light from flowing in. What you need is a window treatment to do the job for you. Unfortunately, most window treatments found in homes these days only do a partial job of blocking light.
If you're a shift worker, then you may need to sleep at odd times of the day. If you live in a noisy neighbourhood, then it can sometimes feel impossible to get the equivalent of a good night's sleep while everyone around you is awake and going about their daily business. You may drop off easily enough, only to be woken up so often that you ultimately find it hard to get back to sleep again.