I think the term came from polyglot books, i.e. books that are written in 2 languages on each side.
Often times for me, the spoken foreign language can be quite fast or filled with colloquials, so I prefer to have subtitles in that language, and also English subtitles. Or in some cases, such as for Japanese voiced games, I’d prefer to have a Japanese subtitles to help me recognize the Kanjis, and additional English subs for translation.
I tried this method on Plex, uploaded dual subtitles for some movies, and it helped me a lot in acquiring new vocabularies. I think Netflix allows users to do the same thing too.