How can YouTube know the content of your videos unless you tell it?
Currently, YouTube does not have the technology to scan your video to transcribe the dialogue (although they are getting close). This is where adding your own subtitles to your videos can help.
Not only will YouTube then know all of the content of your video (boosting your SEO) but it also adds the much-needed accessibility for your customers who are hard of hearing. If you would like to break into a new market you can also pay to have your subtitles translated into a foreign language, right on the YouTube website.