When you go to the movies, there's nothing worse than the theater forgetting to dim the lights. You can't see the movie, so everyone starts groaning.
It's the same when you're watching a YouTube or Netflix video. It's not that easy to see if the rest of your screen is lit up with emails and other websites.
Which is the idea behind Turn Off the Lights. It lets you dim your computer screen, except for the video. (See photo.)
All of a sudden, the video you're watching takes center stage. Everything else on the screen fades into the background, as it should.