joshshifman is right. On the Facebook and Instagram mobile apps, where tons of videos are consumed today, videos play without the sound turned on, and there's no way for creators to have the captions show up without burning them directly into the videos. There's a lot of research (https://www.facebook.com/business/news/updated-features-for-...) that shows that subtitled videos perform better for marketers and content creators.

Not OP, but my guess is that videos autoplay on feeds (FB and twitter for example) but without sound. Therefore, burning in the subtitles is kind of a hacky way to engage users.