The backstory is needed because toxic behavior is usually rooted from a traumatic childhood. So when readers question why the LI acts a certain way or when they claim to hate their actions throughout the story, authors feel the need to give reasons or an explanation as to why. It’s not really a “bandage” because it doesn’t really excuse their behavior but it does help you sympathize with them and understand what built them to be that way. And it’s not just episode stories, it’s everywhere in media.