Turning the subtitles on is useful when you have to turn the volume down at night. Believe it or not it helps you hear what's being said.

I also use it with the foreign language I am trying to learn. Both audio and subtitles set to the target language is useful. Though as someone has already mentioned, the two don't always match which is both frustrating and educational. You get to hear a synonym or a different way of saying the same thing. It helps fill in the blanks sometimes.