Although broadcast video, Blu-ray etc. has functionality to display captions through an associated timecoded text file, other areas of the system often run video players with reduced functionality that do not allow developers or platform holders to add caption files. For example game trailers in the store.
This makes them inaccessible not only people with permanent physical hearing loss, but also temporary and situational impairments such as playing in a noisy room, on mute to avoid waking the baby, or difficulty understanding alien robotic accents on top of a loud music track.
Trailer for Mass Effect Andromeda, with captions displayed through the YouTube player’s captioning functionality