To get your attention. I know cause when I find one of these videos the captions feel like a flashing light telling you something important is happening, it's worse than youtube's shitty clickbait because is baked into the video so you can try looking away, but it's so distracting it genuely feels you're fighting the video by ignoring them.

They are also pointless. One word at a time doesn't have any benefit over normal captions, it's just distraction.

They turned an accessibility feature to an inaccessibility one.