Subtitles are great for viewers who speak a different language than the one spoken in your video.
With subtitles, your video content can reach a far greater number of people around the world. Subtitles are also useful for people who speak the same language with a different and very strong accent -for example, a British person watching an Australian film.
But subtitles don't provide hearing-impaired viewers with the full sound experience. Why? Because subtitles leave out ambient sounds that are critical to conveying a message.
An obvious example of a crucial ambient sound would be door squeaking in a horror movie. You know something scary is coming, but the person who can't hear doesn't have any idea what's going on.
When watching other types of videos, you probably don't think about the ambient sounds in them. But these sounds give important clues to stories.
Think about watching a newscast by a war correspondent. It doesn't have the same impact without the sound of bombs or gunfire in the background.
In the same vein, a movie showing an interior of an apartment may have a lot of traffic noises and shouting outside. These sounds indicate that the apartment is in a metropolitan area.
Captioning shows not only dialogue and narration to the viewer, but also song lyrics and these sounds that make every video more meaningful.