Problem: Google cannot identify accurately any writen or spoken contents of your video.
Currently: In spite of your video being your page main contents, without subtitles each video page only gets indexed by Google from its title and text surrounding your video.
Solution: Search engines can actually read your video subtitles and transcript.
Action: Subtitle your video, and add your video transcription, and let all search engines crawl again your site.
Result: Your site position on search engine result pages (SERP) is now optimized weighting in your actual video contents.
Say you have a video in English.
Nowadays you have leads to your site from Google.com, from keywords in English, from English speaking people.
Say you add subtitles in German, Spanish, French, Italian, Russian, Chinese. And add a transcript of your video in English.
You can get:
All of these extra leads will find your site thanks to indexing not only your page title and headers, but the whole video contents.