I think they only put them in when there’s a long period with no captions. Usually at the beginning or end of a video, but I’ve had them during a silent/dialogue free scene in the middle of a movie before.

That’s not actually true (according to Wikipedia and a few other sources, at least). The ‘blood is thicker than water’ saying has been around for centuries in various forms with the current meaning, and the covenant/womb variation is relatively recent, and mostly stems from a few books that claim that it’s the ‘real meaning’ without sources or proof.