Specifically, because some pirates will put advertisements for other illicit services at the beginning or end of movies and tv shows in its subtitle data where

Specifically, because some pirates will put advertisements for other illicit services at the beginning or end of movies and tv shows in its subtitle data where there's a suitable gap. Usually those gaps have silence. Companies incorporating subtitle data as transcription source of truth training data will thus train their models to output facsimiles of these messages whenever they're encountering prolonged stretches of silence.